Please review the following:
Are you creating a user that was previously deleted? If so, the system will not allow you to recreate as the user is stored in the backend. If you need to reinstate a user that was deleted by mistake, you will need to contact support for assistance.
For security purposes, the system requires that each user account have a unique email address and username assigned.
If you are still having issues after reviewing the above items, contact us at firstname.lastname@example.org and please include as much detail as possible.