Why can't I delete one Validic User ID under my organization?
There is a possibility that the Validic User ID you are trying to delete is the Administrative User. This user is a default developer's user account created to identify a specific Organization by default. It is a requisite in order to create an organization successfully. As such, this user can not be deleted.
We suggest using this user as a reference, but should you need testing, you may provision a new Validic user in order to avoid any issues.