Diagnosis
In the report row hierarchy &V2&, row &V1& is referenced as a totalsrow in no other row. As a result, row &V1& is a basis row of the basisrow hierarchy.
However, you must make an assignment to a cost element group for basisrows of the basis row hierarchy.
This is not included in the definition of row &V1& however.

Procedure
Use transaction BWOM01 to correct the report row definition for reportrow hierarchy &V3&.
Reference in another basis row to the totals row &V1&, or assign
a cost element group to the basis row &V1&.