Like Bluvirus said before, Balmung is locked because it's a congested server. You'll be able to transfer only when SE decides to change it to standard. Maybe they'll unlock Balmung in this next maintenance.

https://na.finalfantasyxiv.com/lodes...nsfer_service/
"Congested World: A crowded, highly populated World. Cannot be selected for new character creation or World transfer destinations."