Trying to create a simple sprinting system

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#1 
--StarterCharacterScripts
local UIS = game:GetService("UserInputService")

local Speed = script.Parent:WaitForChild("Humanoid").WalkSpeed

UIS.InputBegan:Connect(function(Input)
	if Input.KeyCode == Enum.KeyCode.LeftShift then
		Speed = 25
	end
end)
UIS.InputEnded:Connect(function(Input)
	if Input.KeyCode == Enum.KeyCode.LeftShift then
		Speed = 16
	end
end)

I don’t get any errors in the console and the script doesn’t do anything.

#2

Haven’t tested the code, but it looks like you are saving the Humanoid.WalkSpeed value rather than the WalkSpeed property itself.

Try changing:

for

local Speed = script.Parent:WaitForChild("Humanoid")

--And then later on the code
Speed.WalkSpeed = 25