for me, the solution was when I updated xcode to the latest version

I had the same issue. It's crazy! I tried over and over to enrol using the Apple Developer App unsuccessfully following the instructions in the Apple docs. Finally, I looked around for some youtube videos and from the first one I saw I realized they rather did it on the website not on the Developer app. So you go to https://developer.apple.com/programs/enroll/ and click on a button like "enroll...", find the option to continue on web rather than going back to the Developer App. There you dont even need to provide an ID. Thats how I did it successfully!