Error 4155 arises when a temporary NI Number has been issued to a member of staff. NI Numbers must not start with GB, BG, NK, IN, TN, NT or ZZ.
Click on the Error and it will take you to the staff member's profile. Click on the Cog next to their name to open up the Personal Details page:
In this example, the NI number starts with ZZ, which renders it an invalid permanent NI Number. Amend this number, then click Submit at the base of the page to save. Return to the Workforce Census area, to recalculate your Census.