Error code: D/W12

This error occurs when an enrolment date supplied by payroll doesn't match the enrolment date already held by People's Pension. It only affects Non-Eligible Jobholders who opted in to the scheme or Entitled Workers who asked to join. 

The best solution is to check the enrolment dates in both payroll and in People's Pension, and to arrange for whichever one is incorrect to be updated accordingly.