Hello, I am trying to make a menu appear once a player has joined, however, my code seems to be wrong, could you help?
My code:
Many thanks 
Your changing a GUIs value on the server. You should be doing it on the client instead as the server shouldn’t be altering a players GUI. I’d either recommend using remote events or cloning the UI to the client when a player first joins
As I know, local scripts cannot detect the Local player in PlayerAdded, as they start running when the player is in the game already. That’s probably why it doesn’t work.
What you can do is, put a script in ServerScriptService, and try using PlayerAdded function there, and achieve the GUI inside the PlayerGui, and enable it.
Though I’m not sure that what is “Mainmenu” and what kind of script (Local script or Server script) you are running this on, so no guarantees.
Try writing it on the client. You don’t even have to add any events because the client runs after the scripts are replicated to the player upon joining the game, so it will automatically open
An alternative to this is to use localscripts in StarterPlayerScripts