Why can I only add an employee to 1 department even though I’m an administrator?

By default, administrators can manage all departments. However, you can set them up like managers/supervisors and restrict the depts they can manage, an example is here, this person, even though an administrator, would only be able to add/edit an employee into the MANAGEMENT dept.

If this is NOT what you want, go to your profile and uncheck “Is this employee a manager/supervisor”