How do you make a weapon do damage only to NPC's and not players?

You could use a Player check using GetPlayerFromCharacter()?

local Players = game:GetService('Players')

Blade.Touched:Connect(function(Hit)
    local Player = Players:GetPlayerFromCharacter(Hit.Parent)
    if not Player then -- makes sure isn't player
    local Humanoid = hit.Parent:FindFirstChild('Humanoid') -- finds humanoid
    if Humanoid then
        -- damage stuff here
    end
end)

How would I implement the script into this knife script?

local parent = script.Parent
local Handle = parent:WaitForChild("Handle")
local Creator = parent:WaitForChild("Creator")
CanDmg = false
Can = true
parent.Equipped:Connect(function()
    Handle.Equip:Play()
    if Creator.Value == nil then
    Creator.Value = parent.Parent end
end)
parent.Unequipped:Connect(function()
    Handle.UnEquip:Play()
end)
repeat wait() until Creator.Value ~= nil
parent.Activated:Connect(function()
    if Handle.Transparency == 1 then return end
    function slash()
        print("hip")
        local creator = Creator.Value
        local Animations = parent:WaitForChild("Animations")
        local AnimChild = Animations:GetChildren()
        local AnimRad = math.random(1,#AnimChild)
        local AnimSel = AnimChild[AnimRad]
        local AP = creator:WaitForChild("Humanoid"):LoadAnimation(AnimSel)
        if Can == true then
        Can = false
        AP:Play()
        wait(0.15)
        CanDmg = true
        Handle.Swing:Play()
        Handle.SlashTrail.Enabled = true
        wait(0.25)
        CanDmg = false
        Handle.SlashTrail.Enabled = false
        wait(0.06)
        Can = true
        end
    end
    slash()
end)
function on(t)
    local h = t.Parent:FindFirstChildOfClass("Humanoid")
    if h ~= nil and CanDmg == true then
        CanDmg = false
        local cre = Creator.Value
        h:TakeDamage(parent.Dmg.Value)
        Handle.Swing:Stop()
        Handle.Hit:Play()
        if h.Health>0 then
        if not h:FindFirstChild("creator") then
            local ov = Instance.new("ObjectValue",h)
            ov.Name = "creator"
            ov.Value = game.Players:WaitForChild(cre.Name)
            else
            local ovs = h:GetChildren()
            for i = 1,#ovs do
            if (ovs[i].Name == "creator") then
            ovs[i].Value = game.Players:WaitForChild(cre.Name) end
            end
            end
        end
    end
end
Handle.Touched:Connect(on)