Sometimes users are given the incorrect roles. User roles can only be changed by enterprise, domain administrators and managers but they cannot assign a role that has more permissions than their own, therefore, a manager cannot assign an administrator tole to a user, because administrators have more permissions than a manager.

Check the user settings that they have been assigned the necessary permissions through their role and that you have the necessary rights to assign them.

If none of these are the cause of the issue or you get an error message, contact Support for assistance.