Diagnosis
You attempted to use a field for summation, but output of sub-totalshas already been defined for this control level. Sub-totals do not makesense at this level for the field concerned because the control levelfield is lower in the logical database hierarchy than the field used asa summation field.

Procedure
Check that the fields you have selected, the sort sequence and thefields you want to use for summation really make sense as a completedefinition.