What causes this issue?

You have either just entered the class type or just entered the class
on a list for searching by classification data.

System Response

The search is not performed.

How to fix this error?

If you want to perform a search by classification data, you must always
enter both the class type and the class.

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.