I would self-hate you personally if that was the problem.
Before joining this forum, I thought the VPN and Mail apps on android were having trouble and tried reporting it, but it turned out I was on the free account. I didn’t get the memo that those were paid subscription services.
Hmm, well, I can’t see much from that error message, but since you say you thought it was free - you did then opt for the paid plan, right? If you aren’t a paid subscriber, then you can’t use their email service.