If the rejection is - 6791 - There must be an entry in one of [FSE101], [LUN64], [SPS26], [FPS26] or [SSE37] as there is an entry in one of the boxes [FSE76], [LUN52], [SPS20], [FPS20] or [SSE31] and there is an entry in [YPD1], and box [YPD1] date + [66 years] is prior to the beginning of the return year or on 6 April of the return year

This error suggests that the client is exempt from paying Class 4 NIC and you have not completed the relevant box on the form to say that client is exempt from paying class 4 NIC. You will need to cross the relevant box on the appropriate supplementary pages of the return.