& is not a former employee

What causes this issue?

When carrying out the initial entry of applicant data, you made an
entry in the <LS>Pers.no.</> field and thereby treated the applicant as
an employee of your enterprise. However, by making an entry in the
<LS>App.gp</>, you have assigned the applicant to an applicant group in
applicant class <LS>AP</> (external applicant).
The system has checked the employee master data and has realized that
the employee whose personnel number you have entered had not left your
enterprise when you entered the initial basic applicant data. The
applicant cannot be considered an external applicant.
The assignment of a personnel number to an applicant group of applicant
class <LS>LS</> (external applicant) is only permissible if the
applicant is a former employee of your enterprise at the time of the
initial entry of basic applicant data.

How to fix this error?

You must assign the employee (who has not yet left the organization) to
applicant group of applicant class <LS>P</> (internal applicant).
Change your entry as relevant in the <LS>App.gp.</> field.

Error message extract from SAP system. Copyright SAP SE.