&1 &2 is locked with a persistent lock by an other process

What causes this issue?

A persistent lock for the requested delivery with document category &v1&
and document number &v2& exists.

System Response

Another process has created a persistent lock for the requested data
record. This means that the data record remains locked until the process
that requested the lock has finished.
This persistent lock is not one of the locks displayed in transaction
SM12.
This persistent lock is requested by certain processes to retain a lock
across transactions. Usually, the lock is automatically released and
deleted after a certain time, when the process finishes.
Some example cases where a persistent lock could occur are:
A user has changed the original ERP outbound delivery and this update is
still pending.

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?

Wait a few minutes and repeat the process. The persistent lock is
usually released again after a short while, when the corresponding
process is complete.

Procedure for System Administrators

For document category PDO you can use report /SCWM/R_DLV_DEL_EXTSYS_LOCK
to view existing locks.
Check whether unprocessed entries exist for the document number. You can
do this in the monitor transaction /SCWM/MON and there in path
Tools->Message Queue
Check for respective document category &v1& and document number &v2&.
Check also for the respective ERP document number .
An unprocessed entry for your delivery / warehouse request may prevent
the persistent lock from being released. Process the entry in question.
The lock entry in /SCWM/R_DLV_DEL_EXTSYS_LOCK should then be removed by
the process automatically.
If there is no corresponding queue entry and you are sure that the
original process that has set the persistent lock does not exists any
more, you can also use /SCWM/R_DLV_DEL_EXTSYS_LOCK to delete a single
lock.
The change key to identify the persistent lock is &v3&
Note: deletion of lock entries can cause inconsistencies if the original
process still exists. Therefore this should only be done if it is
ensured that there is no original process that requires the persistent
lock any more.

Error message extract from SAP system. Copyright SAP SE.