Ultimatepay option not working?

Are you talking about using UltimatePoints?

You need a minimum of 899 UP (UltimatePoints) in order to use that method. Otherwise you'd need to use the other formats presented to purchase such as the Ultimate Game Card (UGC) prepaid gaming card.

How many UltimatePoints do you have?

I think it initially, automatically goes to your UP balance if you have some; you can always decline to use it by just clicking "No, do not use my UltimatePoints now" beneath the purple submit/use button.
From there at the top right of that small window you can see your UP balance.

Here's some more info about UltimatePay:
https://site.ultimatepay.com/
https://site.ultimatepay.com/tos

Please elaborate further if I wasn't helpful.