I'm trying to add a new starter, but it says I don't have any licenses left. I only have [x] employees, so there should be spare licenses.
Employees in Joining, Active and Former status all use up a license. You can check your usage by going to Licensing > Usage.
Archived employees do not use a license, so you may wish to archive a former employee. You can do this by going to Organisation > Employees > Former.
If not, you can purchase more licenses by going to the Licensing page.