I believe it was because it was very close to departure. The FAT - PHX flight was scheduled to depart at 7.15pm and I was buying the ticket at 6.30pm.

The AA app said that the itinerary couldn't be booked online. The agent on the phone also said he couldn't book it. It could only be booked at the counter at the airport.

I am not sure what the cut-off is for online or phone ticketing but clearly earlier than at the ticket counter.