Same issue here, on a newly opened enrolled account.

Hi! I am having the same issue. I enrolled in the Apple Developer Program 24 hours ago. I can access the Apple Developer Portal and the subscription is active, but I can not access App Store Connect. Error is ITC.signin.error.invalidUser I raised multiple requests to Apple Developer Support and I have received responses promising to call me within the day. Received one call, robotic voice says to press 1 to accept the call and talk to an agent. Then silence for a few seconds and the call gets closed from their side... then I get an email saying they could not reach out to me and they will retry later. They did not retry (yet). Will provide updates on my case too.