you've already got an account, click !autism@lemmy.world and then the Subscribe button
are you using a mobile app or just the lemmy.world website?
I dont know what Instance I am
you are on lemmy.world
but am being asked for a instance
it sounds like you were trying to Subscribe without being logged in
so I'm guessing you have your login in an app instead of the website and then you opened the community in your web browser instead of in the app which already had your login
the link I provided should work in your app or wherever you're commenting from