This error message appears because the Personnel No in CorePersonnel has been appointed or commenced in a Post therefore cannot be unlinked at this stage.
To resolve this issue:
Support will need to run some scripts on the database. Therefore please raise a new case online and reference the title of this article and please include details of the Personnel No and the post that needs to be reverted back to not be attached to an employee.
Unlink this Personnel No from the Applicant No using the correct procedure.
Once this has been completed - please follow the steps outlined in PeopleXD People Management: Deleting an employee set up in error when updating the incorrectly created Personnel No.
You should then be able to Link the existing Applicant to the correct Personnel No.