Its only 2 reasons, that account has been banned/nefarious actor either one will also get you shut down. Maybe the address has a typo, double check and if you got it correct do not use that VA.
I have seen this message posted by several sellers over the past few days.
My advice is that you add a secondary user (e-mail address different from your main seller account e-mail) into your own Seller Account, and then use this same e-mail address for user permissions in any other account.
Hope this helps! Feel free to post here whenever you need help
The error message you’re receiving, “The secondary user cannot be added to this seller account with this email, please use an alternative email,” suggests that there may be restrictions or limitations associated with the email address you’re attempting to use for the VA.