I had that happen to me, I would be able to get to the payment screen, click pay, and it would tell me an error occurred.

I call Reservations, they can't see any award space, transferred to supervisor, she can see it on the website but can't book, she transfers me to the website help desk, who after an hour decides that the website is showing available seats that doesn't exist.

After some loud complaints to corporate, they gave me the seats the website was showing.