How I base this sword's damage off a playerstat?

The problem here is your Power Value is not being updated as the value changes because the variable is never updated throughout the script to fix this you could simply do this

I have changed the Damage = Variables throughout the script you can check them out.

Tool = script.Parent
Handle = Tool:WaitForChild("Handle")

function Create(ty)
    return function(data)
        local obj = Instance.new(ty)
        for k, v in pairs(data) do
            if type(k) == 'number' then
                v.Parent = obj
            else
                obj[k] = v
            end
        end
        return obj
    end
end

local BaseUrl = "rbxassetid://"
local Player = script.Parent.Parent.Parent
Players = game:GetService("Players")
Debris = game:GetService("Debris")
RunService = game:GetService("RunService")

DamageValues = {
    BaseDamage =  0.01 ,
    SlashDamage = .10,
    LungeDamage = .30
}

--For R15 avatars
Animations = {
    R15Slash = 522635514,
    R15Lunge = 522638767
}

Damage = Player:WaitForChild('leaderstats'):WaitForChild('Power').Value * DamageValues.BaseDamage

Grips = {
    Up = CFrame.new(0, 0, -1.70000005, 0, 0, 1, 1, 0, 0, 0, 1, 0),
    Out = CFrame.new(0, 0, -1.70000005, 0, 1, 0, 1, -0, 0, 0, 0, -1)
}

Sounds = {
    Slash = Handle:WaitForChild("SwordSlash"),
    Lunge = Handle:WaitForChild("SwordLunge"),
    Unsheath = Handle:WaitForChild("Unsheath")
}

ToolEquipped = false

--For Omega Rainbow Katana thumbnail to display a lot of particles.
for i, v in pairs(Handle:GetChildren()) do
    if v:IsA("ParticleEmitter") then
        v.Rate = 20
    end
end

Tool.Grip = Grips.Up
Tool.Enabled = true

function IsTeamMate(Player1, Player2)
    return (Player1 and Player2 and not Player1.Neutral and not Player2.Neutral and Player1.TeamColor == Player2.TeamColor)
end

function TagHumanoid(humanoid, player)
    local Creator_Tag = Instance.new("ObjectValue")
    Creator_Tag.Name = "creator"
    Creator_Tag.Value = player
    Debris:AddItem(Creator_Tag, 2)
    Creator_Tag.Parent = humanoid
end

function UntagHumanoid(humanoid)
    for i, v in pairs(humanoid:GetChildren()) do
        if v:IsA("ObjectValue") and v.Name == "creator" then
            v:Destroy()
        end
    end
end

function Blow(Hit)
    if not Hit or not Hit.Parent or not CheckIfAlive() or not ToolEquipped then
        return
    end
    local RightArm = Character:FindFirstChild("Right Arm") or Character:FindFirstChild("RightHand")
    if not RightArm then
        return
    end
    local RightGrip = RightArm:FindFirstChild("RightGrip")
    if not RightGrip or (RightGrip.Part0 ~= Handle and RightGrip.Part1 ~= Handle) then
        return
    end
    local character = Hit.Parent
    if character == Character then
        return
    end
    local humanoid = character:FindFirstChildOfClass("Humanoid")
    if not humanoid or humanoid.Health == 0 then
        return
    end
    local player = Players:GetPlayerFromCharacter(character)
    if player and (player == Player or IsTeamMate(Player, player)) then
        return
    end
    UntagHumanoid(humanoid)
    TagHumanoid(humanoid, Player)
    humanoid:TakeDamage(Damage) 
end


function Attack()
    Damage = Player:WaitForChild('leaderstats'):WaitForChild('Power').Value * DamageValues.SlashDamage
    Sounds.Slash:Play()

    if Humanoid then
        if Humanoid.RigType == Enum.HumanoidRigType.R6 then
            local Anim = Instance.new("StringValue")
            Anim.Name = "toolanim"
            Anim.Value = "Slash"
            Anim.Parent = Tool
        elseif Humanoid.RigType == Enum.HumanoidRigType.R15 then
            local Anim = Tool:FindFirstChild("R15Slash")
            if Anim then
                local Track = Humanoid:LoadAnimation(Anim)
                Track:Play(0)
            end
        end
    end 
end

function Lunge()
    Damage = Player:WaitForChild('leaderstats'):WaitForChild('Power').Value * DamageValues.LungeDamage

    Sounds.Lunge:Play()

    if Humanoid then
        if Humanoid.RigType == Enum.HumanoidRigType.R6 then
            local Anim = Instance.new("StringValue")
            Anim.Name = "toolanim"
            Anim.Value = "Lunge"
            Anim.Parent = Tool
        elseif Humanoid.RigType == Enum.HumanoidRigType.R15 then
            local Anim = Tool:FindFirstChild("R15Lunge")
            if Anim then
                local Track = Humanoid:LoadAnimation(Anim)
                Track:Play(0)
            end
        end
    end 
    --[[
    if CheckIfAlive() then
        local Force = Instance.new("BodyVelocity")
        Force.velocity = Vector3.new(0, 10, 0) 
        Force.maxForce = Vector3.new(0, 4000, 0)
        Debris:AddItem(Force, 0.4)
        Force.Parent = Torso
    end
    ]]

    wait(0.2)
    Tool.Grip = Grips.Out
    wait(0.6)
    Tool.Grip = Grips.Up

    Damage = Player:WaitForChild('leaderstats'):WaitForChild('Power').Value * DamageValues.SlashDamage
end

Tool.Enabled = true
LastAttack = 0

function Activated()
    if not Tool.Enabled or not ToolEquipped or not CheckIfAlive() then
        return
    end
    Tool.Enabled = false
    local Tick = RunService.Stepped:wait()
    if (Tick - LastAttack < 0.2) then
        Lunge()
    else
        Attack()
    end
    LastAttack = Tick
    --wait(0.5)
    Damage = Player:WaitForChild('leaderstats'):WaitForChild('Power').Value * DamageValues.BaseDamage
    local SlashAnim = (Tool:FindFirstChild("R15Slash") or Create("Animation"){
        Name = "R15Slash",
        AnimationId = BaseUrl .. Animations.R15Slash,
        Parent = Tool
    })

    local LungeAnim = (Tool:FindFirstChild("R15Lunge") or Create("Animation"){
        Name = "R15Lunge",
        AnimationId = BaseUrl .. Animations.R15Lunge,
        Parent = Tool
    })
    Tool.Enabled = true
end

function CheckIfAlive()
    return (((Player and Player.Parent and Character and Character.Parent and Humanoid and Humanoid.Parent and Humanoid.Health > 0 and Torso and Torso.Parent) and true) or false)
end

function Equipped()
    Character = Tool.Parent
    Player = Players:GetPlayerFromCharacter(Character)
    Humanoid = Character:FindFirstChildOfClass("Humanoid")
    Torso = Character:FindFirstChild("Torso") or Character:FindFirstChild("HumanoidRootPart")
    if not CheckIfAlive() then
        return
    end
    ToolEquipped = true
    Sounds.Unsheath:Play()
end

function Unequipped()
    Tool.Grip = Grips.Up
    ToolEquipped = false
end

Tool.Activated:Connect(Activated)
Tool.Equipped:Connect(Equipped)
Tool.Unequipped:Connect(Unequipped)

Connection = Handle.Touched:Connect(Blow)

Tool = script.Parent
Handle = Tool:WaitForChild("Handle")

function Create(ty)
    return function(data)
        local obj = Instance.new(ty)
        for k, v in pairs(data) do
            if type(k) == 'number' then
                v.Parent = obj
            else
                obj[k] = v
            end
        end
        return obj
    end
end

local BaseUrl = "rbxassetid://"

Players = game:GetService("Players")
Debris = game:GetService("Debris")
RunService = game:GetService("RunService")

local plr = game.Players:GetPlayerFromCharacter(script.Parent.Parent.Parent)
plrstat = plr.leaderstats.Power.Value

DamageValues = {
    BaseDamage =  plrstat * 0.01,
    SlashDamage = plrstat * .10,
    LungeDamage = plrstat * .30
}



--For R15 avatars
Animations = {
    R15Slash = 522635514,
    R15Lunge = 522638767
}

Damage = DamageValues.BaseDamage

Grips = {
    Up = CFrame.new(0, 0, -1.70000005, 0, 0, 1, 1, 0, 0, 0, 1, 0),
    Out = CFrame.new(0, 0, -1.70000005, 0, 1, 0, 1, -0, 0, 0, 0, -1)
}

Sounds = {
    Slash = Handle:WaitForChild("SwordSlash"),
    Lunge = Handle:WaitForChild("SwordLunge"),
    Unsheath = Handle:WaitForChild("Unsheath")
}

ToolEquipped = false

--For Omega Rainbow Katana thumbnail to display a lot of particles.
for i, v in pairs(Handle:GetChildren()) do
    if v:IsA("ParticleEmitter") then
        v.Rate = 20
    end
end

Tool.Grip = Grips.Up
Tool.Enabled = true

function IsTeamMate(Player1, Player2)
    return (Player1 and Player2 and not Player1.Neutral and not Player2.Neutral and Player1.TeamColor == Player2.TeamColor)
end

function TagHumanoid(humanoid, player)
    local Creator_Tag = Instance.new("ObjectValue")
    Creator_Tag.Name = "creator"
    Creator_Tag.Value = player
    Debris:AddItem(Creator_Tag, 2)
    Creator_Tag.Parent = humanoid
end

function UntagHumanoid(humanoid)
    for i, v in pairs(humanoid:GetChildren()) do
        if v:IsA("ObjectValue") and v.Name == "creator" then
            v:Destroy()
        end
    end
end

function Blow(Hit)
    if not Hit or not Hit.Parent or not CheckIfAlive() or not ToolEquipped then
        return
    end
    local RightArm = Character:FindFirstChild("Right Arm") or Character:FindFirstChild("RightHand")
    if not RightArm then
        return
    end
    local RightGrip = RightArm:FindFirstChild("RightGrip")
    if not RightGrip or (RightGrip.Part0 ~= Handle and RightGrip.Part1 ~= Handle) then
        return
    end
    local character = Hit.Parent
    if character == Character then
        return
    end
    local humanoid = character:FindFirstChildOfClass("Humanoid")
    if not humanoid or humanoid.Health == 0 then
        return
    end
    local player = Players:GetPlayerFromCharacter(character)
    if player and (player == Player or IsTeamMate(Player, player)) then
        return
    end
    UntagHumanoid(humanoid)
    TagHumanoid(humanoid, Player)
    humanoid:TakeDamage(Damage) 
end


function Attack()
    Damage = DamageValues.SlashDamage
    Sounds.Slash:Play()

    if Humanoid then
        if Humanoid.RigType == Enum.HumanoidRigType.R6 then
            local Anim = Instance.new("StringValue")
            Anim.Name = "toolanim"
            Anim.Value = "Slash"
            Anim.Parent = Tool
        elseif Humanoid.RigType == Enum.HumanoidRigType.R15 then
            local Anim = Tool:FindFirstChild("R15Slash")
            if Anim then
                local Track = Humanoid:LoadAnimation(Anim)
                Track:Play(0)
            end
        end
    end 
end

function Lunge()
    Damage = DamageValues.LungeDamage

    Sounds.Lunge:Play()

    if Humanoid then
        if Humanoid.RigType == Enum.HumanoidRigType.R6 then
            local Anim = Instance.new("StringValue")
            Anim.Name = "toolanim"
            Anim.Value = "Lunge"
            Anim.Parent = Tool
        elseif Humanoid.RigType == Enum.HumanoidRigType.R15 then
            local Anim = Tool:FindFirstChild("R15Lunge")
            if Anim then
                local Track = Humanoid:LoadAnimation(Anim)
                Track:Play(0)
            end
        end
    end 
    --[[
    if CheckIfAlive() then
        local Force = Instance.new("BodyVelocity")
        Force.velocity = Vector3.new(0, 10, 0) 
        Force.maxForce = Vector3.new(0, 4000, 0)
        Debris:AddItem(Force, 0.4)
        Force.Parent = Torso
    end
    ]]

    wait(0.2)
    Tool.Grip = Grips.Out
    wait(0.6)
    Tool.Grip = Grips.Up

    Damage = DamageValues.SlashDamage
end

Tool.Enabled = true
LastAttack = 0

function Activated()
    if not Tool.Enabled or not ToolEquipped or not CheckIfAlive() then
        return
    end
    Tool.Enabled = false
    local Tick = RunService.Stepped:wait()
    if (Tick - LastAttack < 0.2) then
        Lunge()
    else
        Attack()
    end
    LastAttack = Tick
    --wait(0.5)
    Damage = DamageValues.BaseDamage
    local SlashAnim = (Tool:FindFirstChild("R15Slash") or Create("Animation"){
        Name = "R15Slash",
        AnimationId = BaseUrl .. Animations.R15Slash,
        Parent = Tool
    })

    local LungeAnim = (Tool:FindFirstChild("R15Lunge") or Create("Animation"){
        Name = "R15Lunge",
        AnimationId = BaseUrl .. Animations.R15Lunge,
        Parent = Tool
    })
    Tool.Enabled = true
end

function CheckIfAlive()
    return (((Player and Player.Parent and Character and Character.Parent and Humanoid and Humanoid.Parent and Humanoid.Health > 0 and Torso and Torso.Parent) and true) or false)
end

function Equipped()
    Character = Tool.Parent
    Player = Players:GetPlayerFromCharacter(Character)
    Humanoid = Character:FindFirstChildOfClass("Humanoid")
    Torso = Character:FindFirstChild("Torso") or Character:FindFirstChild("HumanoidRootPart")
    if not CheckIfAlive() then
        return
    end
    ToolEquipped = true
    Sounds.Unsheath:Play()
end

function Unequipped()
    Tool.Grip = Grips.Up
    ToolEquipped = false
end

Tool.Activated:Connect(Activated)
Tool.Equipped:Connect(Equipped)
Tool.Unequipped:Connect(Unequipped)

Connection = Handle.Touched:Connect(Blow)

where it errors out this will get the local player (the player that holds the sword) and look inside their leaderstats and change the values of the damages