How to resolve the 'User Already Exists or Was Previously Deleted' error when adding a user in Vtiger?

The user you are trying to add was previously in the CRM users list but is now deactivated or suspended.
Please follow these steps to reactivate an inactive or suspended user:

1. Click the User Menu on the top right corner of the screen.
2. Click the Settings button.
3. Look for the User Management section.
4. Select Users.
5. Click the Inactive Users tab. 
6. Hover over the user that you want to reactivate.
7. Click the More icon.
8. Select the Reactivate User option.
9. Click Yes to confirm.