I've encountered the same issue.
For a workaround, I was able to submit the build to TestFlight using the AppStore Connect app on my iPhone instead of using the website.
greensopinion
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
Last seen
User for