attempt to index nil with 'Position' ?

i think you forgot to make the variable myRoot

local myRoot = script.Parent:WaitForChild("HumanoidRootPart")

function findTarget()
    local dist = 200
    local target = nil 
    for i,v in ipairs(workspace:GetChildren()) do
        local human = v:FindFirstChild("Humanoid")
        local torso = v:FindFirstChild("Torso") or v:FindFirstChild("HumanoidRootPart")
        if human and torso and v.Name ~= script.Parent.Name then
            if (myRoot.Position - torso.Position).Magnitude < dist and human.Health > 0 then
                target = torso
                dist = (myRoot.Position - torso.Position).Magnitude
            end
        end
    end
    return target
end

function checkSight(target)
    local ray = Ray.new(myRoot.Position, (target.HumanoidRootPart.Position - myRoot.Position).Unit * 200)
    local hit,position = workspace:FindPartOnRayWithIgnoreList(ray, {script.Parent})
    if hit then
        if hit:IsDescendantOf(target.Parent) then
            return true
        end
    end
    return false
end

Nope my friend I already made the variable you need the whole script ? Here it's :

-- Main body parts
local myRoot = script.Parent:WaitForChild("HumanoidRootPart")
local myHuman = script.Parent:WaitForChild("Humanoid")
local rightHand = script.Parent:WaitForChild("RightHand")
local upperTorso  =script.Parent:WaitForChild("UpperTorso")

-- Weapon
local sword = script.Parent:WaitForChild("Sword")
local swordweld = sword:WaitForChild("SwordWeld")

--Sounds
local lungeSound = sword:WaitForChild("Lunge")
local slashSound = sword:WaitForChild("Slash")
local unsheathSound = sword:WaitForChild("Unsheath")

--AI booleans
local attackCool = true
local swordHolstered = false
local chasing = false
local pathFailcount = 0

--Animation
local leftSlash = script.Parent:WaitForChild("LeftSlash")
local leftSlashAnimation = myHuman:LoadAnimation(leftSlash)
leftSlashAnimation.Priority = Enum.AnimationPriority.Action

local rightSlash = script.Parent:WaitForChild("RightSlash")
local rightSlashAnimation = myHuman:LoadAnimation(rightSlash)
rightSlashAnimation.Priority = Enum.AnimationPriority.Action

--Reaspawn
local clone = script.Parent:Clone()

function getUnstuck()
    myHuman:Move(Vector3.new(math.random(-1,1), 0, math.random(-1,1)))
    wait(0.3)
end

function wander()
    local xRand = math.random(-50,50)
    local zRand = math.random(-50,50)
    local goal = myRoot.Position + Vector3.new(xRand, 0, zRand)

    local PathfindingService = game:GetService("PathfindingService"):CreatePath()
    PathfindingService:ComputeAsync(myRoot.Position, goal)
    if PathfindingService.Status == Enum.PathStatus.Success then
        pathFailcount = 0
        local waypoints = PathfindingService:GetWayPoints()
        for i,v in ipairs(waypoints) do
            if v.Action == Enum.PathWaypointAction.Jump then
                myHuman.Jump = true
            end
            myHuman:MoveTo(v.Position)
            if i % 5 == 0  then
                if findTarget() then
                    break
                end
            end
            local moveSuccess = myHuman.MoveToFinished:Wait()
            if not moveSuccess then
                break
            end
        end
    else
        pathFailcount = pathFailcount + 1
        if pathFailcount == 10 then
            pathFailcount = 0
            getUnstuck()
        end
    end
end

function findTarget()
    local dist = 200
    local target = nil 
    for i,v in ipairs(workspace:GetChildren()) do
        local human = v:FindFirstChild("Humanoid")
        local torso = v:FindFirstChild("Torso") or v:FindFirstChild("HumanoidRootPart")
        if human and torso and v.Name ~= script.Parent.Name then
            if (myRoot.Position - torso.Position).Magnitude < dist and human.Health > 0 then
                target = torso
                dist = (myRoot.Position - torso.Position).Magnitude
            end
        end
    end
    return target
end
--  THE ERROR HAPPENS HERE ....
function checkSight(target)
    local ray = Ray.new(myRoot.Position, (target.Position - myRoot.Position).Unit * 200)
    local hit,position = workspace:FindPartOnRayWithIgnoreList(ray, {script.Parent})
    if hit then
        if hit:IsDescendantOf(target.Parent) then
            return true
        end
    end
    return false
end
-- BETWEEN THIS COMMENT ....
function checkHeight(target)
    if math.abs(myRoot.Position.Y - target.Position.Y) < 3 then
        return true
    end
    return false
end

function checkDirect(target) 
    if checkSight() and checkHeight() and (myRoot.Position - target.Position).Magnitude < 30 then
        return true
    end
    return false
end

function pathToTarget(target)
    local path = game:GetService("PathfindingService"):CreatePath()
    path:ComputeAsync(myRoot.Position, target.Position)
    if path.Status == Enum.PathStatus.Success then
        pathFailcount = 0
        local waypoints = path:GetWayPoints()
        for i,v in ipairs(waypoints) do
            if v.Action == Enum.PathWaypointAction.Jump then
                myHuman.Jump = true
            end
            spawn(function()
                wait(0.5)
                if myRoot.Position.Y < myHuman.WalkToPoint.Y and chasing == false then
                    myHuman.Jump = true
                end
            end)
            myHuman:MoveTo(v.Position)
            local moveSuccess = myHuman.MoveToFinished:Wait()
            if not moveSuccess then
                break
            end
            if i % 5 == 0 then
                if checkDirect(target) then
                    break
                end
                if target ~= findTarget() then
                    break
                end
            end
            if (waypoints[#waypoints].Position - target.Position).Magnitude > 30 then
                break
            end
        end
    else
        pathFailcount = pathFailcount + 1
        if pathFailcount > 10 then
            pathFailcount = 0
            getUnstuck()
        end
    end
end

function chase(target)
    myHuman.WalkSpeed = 20
    chasing = true
    myHuman:MoveTo(target.Position)
end

function main()
    chasing = false
    myHuman.WalkSpeed = 16
    local target = findTarget()
    if target then
        local targetDistance = (myRoot.Position - target.Position).Magnitude
        if checkDirect(target) then
            if targetDistance > 3 then
                chase(target)
            end
        elseif checkSight(target) and math.abs(myRoot.Position.X - target.Position.X) < 1 and
            math.abs(myRoot.Position.Z - target.Position.Z) < 1  then
            getUnstuck()
        else
            pathToTarget(target)
        end
    else
        wander()
    end
end

while wait(0.1) do
    main()
end

hey you! have you ever heard of enes? if you are in trouble, better call enes!