I am trying to book airfare, but when I go to reserve a ticket I get the following error message:

There are plenty of seats on both legs of the trip, and when I go to search again the flights are still there so they're valid. BCD travel can also see the flights are valid when I call. I have tried selecting these flights without selecting seats as well and that doesn't resolve the issue. I have to reserve these flights over the phone and pay a $45 fee to do so, so any help on what's going on here would be much appreciated.