i tried doing this mutable ways but, seems to not be working.
game.StarterGui.MainMenu.Background.Visible = false
Do you want it to disappear when a player presses a button or something?
In any case, the problem is that you must access the GUI frame of the specific player.
game.Players.LocalPlayer.MainMenu.Background.Visible = false
It must be in a LocalScript.
Edit:
Add a close button for the menu, and add a LocalScript in the button:
script.Parent.MouseButton1Click:connect(function() game.Players.LocalPlayer.PlayerGui.Mainmenu.Background.Visible = false end)