Internal error: Set & does not exist.

What causes this issue?

A set used in a report cannot be found when reading a report prior to
generation or for report maintenance.
The report is inconsistent as a result.

System Response

The report cannot be processed.

How to fix this error?

This inconsistency probably occurred as a result of an incorrect report
transport (without dependent objects). In this case, repeat the
transport (with dependent objects).

Error message extract from SAP system. Copyright SAP SE.