We were having this issue, and despite multiple attempts to speak with seller support they were unable to help. I think I have found a fix for this.

1. It has nothing to do with the email address.

2. When onboarding some colleagues, we have found that once they get to the point of reaching the Amazon account home screen, there will be an action to set up 2 factor authentication.

3. Despite already setting this up in the initial account set up, it requires you to do this again. It feels like you are repeating the action.

4. Refresh user permissions. No more error!

If it had asked for this, instead of saying that the issue was with the email address, it would have saved me a lot of time and effort!

I hope this helps someone!