Change request &2 locked by user &1

What causes this issue?

A change request is locked when you change its processing status and it
is processed directly (for example, by adding new objects). A change
request is also locked when new objects or hierarchy relationships are
added. This occurs in single processing, data transfer (upload of master
data, language-dependent texts or hierarchies), or in collective
processing.
As a consequence, if a change request is used for multiple entity types,
these cannot be processed simultaneously because each locks the same
change request.

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?

You can use transaction SM12 to view the locks. The lock table is called
USMD120C and the lock arguments are the client and the technical key of
the change request.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.