I ran into this same issue a little over a week ago booking my group's room. In my case, it looked like it was a minor site glitch, as I was able to go through with the booking process after waiting for a couple of hours.
"You're an adult, and you can put on a costume whenever you want. You don't need permission anymore. If you wake up next Tuesday, and you feel like being Batman, go for it."
- Lewis Black
Anime NebrasKon 2015
Anime Los Angeles 2016(?)