Any worlds listed in the server status list as "congested worlds" are closed for the foreseeable future. Currently, Balmung is the only NA world with that status, so you will need to choose a different world.

Any other world, if you attempt to create a character and it won't let you, it means that at that particular moment, the server was too busy for a new character. You might be able to get you in if you come back during slower hours.