This code means that the auto enrolment date supplied by payroll does not match the auto enrolment date already held by People's Pension.
The pensionsync error report shows what date was supplied in the payroll data and also confirms what date People's Pension currently hold, enabling payroll to check which is the correct date.
If People's Pension hold an incorrect date their support team can update it for you. This might happen if the employee was enrolled before the staging date and the system has never been updated with the first date they were assessed as an eligible jobholder, or it could simply be that the incorrect date was sent in the first place and needs correcting. There is guidance around correcting AE dates on People's Pension's website here: https://thepeoplespension.co.uk/help/knowledgebase/can-an-ae-date-be-changed-as-it-was-uploaded-incorrectly/
If it's payroll that holds the incorrect date, you should update this in your payroll software so that the data supplied in future matches the correct date held by People's Pension.