AK = Air Asia, they don't fly in the USA, suspect you're looking for AS.

I had something similar happen recently trying to book a G3 flight on miles. It actually put it on hold and was the last seat with miles available. Check your Aadvantage account, it may be there awaiting payment, that's what happened to me. So when I tried various browsers it still showed a seat available, but would error out when trying to book it. Then when I checked my current itineraries in my account it was actually there and just had to complete the booking.