HMRC do not accept negative year-to-date values in an FPS. This is an HMRC error message, not ours. The FPS is being submitted, and they are rejecting it.
You cannot send negative year-to-date values in every/most fields in an RTI full payment submission.
Investigate the Pay item that is causing the negative YTD value using the Employee Analyse functionality (search “Pay run Analyse tab”). Once this has been identified, correct the pay to ensure the YTD value is positive.