Deleted users are not permanently deleted from your database - they're still stored on the back-end of your account. This error message indicates that the email address you're trying to assign to the new user is still assigned to one of your deleted admins.
Instead of creating a new user account with this email address, the CampSite Support team must reinstate the previous account that was deleted. To request that an admin user's account be reinstated, send an email to CampSite Support via firstname.lastname@example.org.
Tip: If the previous user account was deleted to revoke access to your database temporarily, in the future, lock them out instead of deleting them. Click here to learn more.