Hi Hun,
I had the same issue earlier today and found a resolution with the help of tech support.
If, like me, you are trying to use a credit card to pay, the billing address might be the issue.
If your billing address for your credit card auto-fills from info saved on your account, trying clearing it and re-entering it. If your address uses two lines (street address & apt# for example), try entering the complete address on one line. It should then make a suggestion for what address to use based on what you entered. It appears to be a glitch, which has now been reported.
I hope this helps! Definitely frustrating.
