attempt to index nil with 'Parent'?

What? Pls Fix This Idk How To

local tool = script.Parent

local function onTouch(partOther)
	
	local humanOther = partOther.Parent:FindFirstChild("Humanoid")
	
	if not humanOther then return end
	
	if humanOther.Parent == tool then return end
	
	humanOther:TakeDamage(25)
end

tool.Activated:Connect(onTouch)

This should happen when the sword touches something, since that will tell the part that touched it

tool.Touched:Connect(onTouch)

edit: also place this script in the weapon's handle

The error is in the line:

tool.Activated:Connect(onTouch)

The function onTouch has one parameter: partOther. When the tool is activated, you called the function onTouch, but you didn't give it any parameters. Therefore partOther would be nil. Fix:

local tool = script.Parent
	 
local function onTouch(partOther)
 
	local humanOther = partOther.Parent:FindFirstChild("Humanoid")

	if not humanOther then return end

	if humanOther.Parent == tool then return end
	 
	humanOther:TakeDamage(25)
end
	 
tool.Activated:Connect(onTouch('whatever partOther is'))