If it will not let you enter a new starter it is possible that:
The maximum number of employees has been reached, in which case refer to the software’s sales team to increase the license.
The number of employees which counts towards the maximum is current employees only (Employee Status = Current).
Leavers are not included.
Sometimes records of casual staff are kept in case they work for the company again. These will be added to the count of current employees unless they are assigned as a Leaver.