6626 - There is an amount in box [3.73]. Boxes [3.24], [3.25], [3.26] must be absent. Please check. ABS-35

Absolute Tax Online Filing Errors SA100, SA800, SA900  

This rejection suggests that you have completed the income into the boxes 3.24, 3.25 and 3.26 as well as the full trade pages on the SA800 tax return. You will need to amend and resubmit to HMRC.