This error suggests that on the tax return you have completed the additional information pages with life insurance gains and not provided a backing schedule.

This is a requirement for filing online to HMRC.

You will therefore need to open the tax return and click the calculator icon to the right of box 4 on page AI1. This will enable you to add and complete a backing schedule showing all gains in the year. This schedule will then be included in the xml file that is filed online to HMRC.