If you are receiving this error it would indicate that two members of staff have the same NI number.
It may be due to having two profiles for one staff member (an inactive one and an active one).
In this case you will need to transfer data from the newer profile to the old inactive one (add the new contract, update contact details, etc), so all the data is on one place, then delete the newer profile. The inactive profile then needs to be made active, by clicking on the cog next to their name and setting Active to Yes (Submit to save).
Alternatively, it could mean that two or more staff have no NI number entered. To remove this error go to the staff profile via Admin > Personnel > Staff - find the member of staff indicated in the error and click onto the black cog next to personal details.
In this area please ensure the NI Number is correct for the member of staff and submit at the bottom to save any changes.