To add some data to the problem space, I'll just mention that I have been trying this since late June 2020 for nearly a week now, using a United States credit union card, and a United States credit card. Both cards showed they had set-aside funds for pre-aproval for $109 charges from Apple, but my account has been stuck in "pending", and when I checked today, the banks had dropped the pre-approvals, showing no charges, and I'm still in "pending". I have talked/chatted now with 4 Apple help people from 3 departments in 5 interactions, and they have all had no idea what is going on and told me to try to make another payment or wait or call a different department. One said the mobile version of the Apple Developer app would allow other payment methods and let them expedite processing, but that requires a recent device with eye/fingerprint recognition, which I don't have and don't want.

Ok, not to make people jealous, but Apple Developer support actually telephoned me today and took my payment over the phone, and it worked, and now my Apple Developer account is working. What I did that seemed to cause this, was to (yesterday) send a polite email to devprograms@apple.com, explaining I had waited about a week after trying to pay using the web site, and providing a link to this forum thread. @pjfisher Yes, there is a phone number (800)275-2273, which you can call but the person I got when I did that admitted he did not know much about Apple Developer Account registration. He thought the chat support would know, but the chat support I talked to did not, and seemed to think the phone support would.

Is there no way to find threads you previously participated in, other than to bookmark them in your web browser or something? The only reference to my past activity seems to be on my profile... which only mentions the number of posts I made, but doesn't mention what or where they are or provide any way to find them.

I'm having the same experience. I was able to hover over the error and get more detail... for about 1/3 of a second, once. Something about not being able to connect (?) or authenticate with the App Store, but I have not been able to induce it to give me that information again. Did you manage to work around this somehow?