(I take it you did call and they said no... I would try and call back a bit later as your original attempted booking may have "blocked" the seats for an hour or two)
You really need to try calling, it is going to be a painful wait but at this point that sounds like your best hope to get an agent on the line and get a definitive whether they can book it and its just an online tool issue or something else. You may be breaking some other rule that isn't immediately obvious. Good luck.