You want to create a new employee, have not yet reached the maximum limit, but the system requests you to upgrade your plan?
Go to the menu item "employees" and filter for "no department" and "not confirmed". Here you might find some employees you have created but not actively used in your shift planning. However they are counted into the plan you have chosen.
As soon as you delete them you will be able to create new employees. Here you can read how to create new employees: