Sales document is currently locked by user &1

What causes this issue?

Sales document &V2& is or was blocked at the stage when the sales
document was processed during the processing step. The user who is
locking or who has locked the the sales document was &V1&.
It is possible that the delivery is or has been simultaneously processed
by another processing step, or by another parallel process.

System Response

The processing step is terminated as parallel processing of a sales
document in different processes can lead to inconsistencies.

How to fix this error?

Wait until the delivery is no longer blocked by another user, and then
restart the processing step again.

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.