PLESK_ERROR: Error: the user exists, but has not been verified. We have sent a new verification email. Check your mailbox.
Antispam is blocking verification emails.
1. Check the spam folder
2. Allow emails from firstname.lastname@example.org in your antispam
In case it doesn't work, please contact Plesk Licensing and Purchase support.
Try to ask the customer use another email address (if licenses aren`t associated). In case customer refuses/did not help - contact Dmitriy Temnov.