What causes this issue?

You specified in the definition of the report or variant that the
system should summarize on the '&V1&' field. However, this was not
possible.

System Response

The data is not summarized on the '&V1&' field.

How to fix this error?

Check for the cause of the error. Possible causes are:
The '&V1&' field is unknown.
The '&V1&' field is not a characteristic.
[Applies only for key figures]: The '&V1&' field is the reference field
for aggregation of key figure '&V2&' which has exception aggregation
&V3& (not "Summation"). If the key figure '&V2&' was summarized based
on the reference field '&V1&', this would cause the key figure to have
nonsensical values. Therefore, it is not possible for the system to
summarize on the '&V1&' field.
The '&V2&' field is dependent on the '&V1&' field, whereby the first
field occurs in the query definition. Due to this dependency of the
characteristics, the '&V2&' field would not be unique without the '
&V1&' field.
In such a case, you should do one of the following:
Remove the '&V2&' field from the query definition.
Add the '&V1&' field to the query definition.

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.