How to set the server spawn

I am creating a minigames server and of course, I want people to spawn in the lobby however when I use "/setworldspawn" it doesn't fix anything. For the lobby I created a flat world using multiverse core and people are just spawning in the original world. I have tried deleting it however it just gives me an error saying "World could not be deleted" Is there anyway for me to set that world as the serverspawn? I am on paper 1.12.2 and the log is empty