Data message &1 is locked

What causes this issue?

The message is already locked by another process; Therefore, it cannot
be locked again. The message is locked if another user is editing it or
if the message is currently processed.

System Response

The selected data message will not be locked again on its persistence
layer. Editing the data messages content is not possible.

How to fix this error?

Wait some time and try to edit the messages again. The message will be
unlocked after the user has saved the edited message or after the
messsage was reprocessed.

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.