https://na.finalfantasyxiv.com/lodestone/worldstatus/

If you look here, Sargantanas is hard locked as congested. It is not possible at any time to create new characters on this world.

This status will not change until most likely 6.55, when the devs will look at changing world statuses across the board.

If you want to play with your friends on Aether now, you'll have to create a character on a non-congested world and then server hop to there.