What it's supposed to do is pop up, when you say "MainMenu/On", for you though. Not everyone else. So what I'm saying is it will pop up for only 1 player, if they say it. Can someone do this? And make it so if you say "MainMenu/Off", it will go not visible again.
if message:lower() == "MainMenu/On" then
gui.Visible = not gui.Visible -- Sets GUI visibility to the opposite of what it was (toggle)
end

2 comments
First issue that comes to mind is that if you make the message lowercase, it won't match the message you've set (which has uppercase letters)
I noticed that as well. On my script, if you want it lowered, I did that for you. If not? Remove my Line 2 and in lines 3 and 6, edit what they say.'