What causes this issue?

The report or variant definition specified that field &V1& should be
used for summarizing. However this was not possible.

System Response

The data was not summarized using the &V1& field.

How to fix this error?

Try to determine the cause of the error. It could be because:
The system does not recognize the &V1& field
The &V1& field is not a key figure

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.