Folks, I've had exactly the same problem. Was tearing my hair out. Eventually I think I solved it.
Firstly - DO NOT select your seats. Just skip that and go straight to payment. You can go back later and do the seats.
Second - If you've recently added a new card but haven't deleted old or expired ones, do that on the payment page - there's a little waste bin symbol beside old cards. Use it to bin them.
Thirdly - In desperation, if the above won't work, delete your CURRENT card and RE-ENTER it in the system.
It worked for me.
Terrible system!
Good Luck.