Employee number is already in use when trying to assign it to an employee

Employee number is already in use when trying to assign it to an employee.

Go to "My Organization → Organization" and search for the employee number in the search field on the left side of the page. This will display lines below showing whether the employee number is registered to an active or deactivated employee.

To free up the number for use, you need to change it on the profile of the person who currently has it. Once updated, you can assign the number to the desired person.