Sadly there are often connexion errors when trying to join a lobby. The only option that doesn't have this issue is "Can't decide", but you can't choose which world you'll play in unfortunately.

I read that you can fix this (at least temporarily) by switching at the Nintendo servers and back to Pretendo from Nimbus, although I still haven't tested this, you may give it a try! But probably it just works because by doing so you simply restart the game.