The script wont work but theres no problems in the output?!?!?

well here is a thing is thing is, all GUIs that are in the StarterGui are cloned into the an individual players PlayerGui. Therefore, to access a gui you'd have to do player.PlayerGui..

so you could rewrite your code like this in a localscript:

local Player = game.Players.LocalPlayer
player.PlayerGui.Menu.MainMenu.Credits.MouseButton1Click:Connect(function() player.PlayerGui.Menu.MainMenu.CreditsFrame.Visible = true end)

or you could just parent the localscript to the Gui and use script.Parent