Classification data can only be maintained using a change number

What causes this issue?

You have allocated the object to a class with
<DS:GLOS.engineering_change_management>engineering change management
</>functionality. This is why you must enter a change number whenever
you change the <DS:GLOS.classification>classification data</> of the
object.

System Response

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

How to fix this error?

Enter a <DS:GLOS.change_number>change number</> to make the change. To
do this, display the change overview to see all the changes that have
been made to the object, including the change number used, and choose a
change number.
For more information on how engineering change management functionality
is used in classification, see document <ZH>CA Classification</>.

Error message extract from SAP system. Copyright SAP SE.