I tried to use LocalPlayer with LocalScript (Because LocalScript is a client side(Also is LocalPlayer)) Here is my code:
local player = game.Players.LocalPlayer
local character = player.Character
local GUIButton = game.StarterGUI.ScreenGui.TextButton
GUIButton.Mouse1ButtonClick:Connect(function()
character.Humanoid.WalkSpeed = character.Humanoid.WalkSpeed + 7
end)

3 comments
I meant MouseButton1Click; Dont mind that typo
And, what's your Problem there?
That the Local Player's Character Property is not working