If you get the error message "The guest could not be reactivated", you'll want to check for an account with the email address minus the deactivated portion as there cannot be multiple guest accounts with the same email.
For example, I have an account with a test email. When I deactivate this account, the email will now read deactivated-[email]. This will allow the email to be reused to create an account. However, if this happens and you then try to reactivate the deactivated guest, you will receive the error message.
If you find a duplicate account with the guest's email, you will need to change the guests email in that duplicate account (for example, add an extra number or letter), then the system will allow you to reactivate the original account.