Document &1 could not be delivered (see long text)

What causes this issue?

Document &V1& could not be delivered. This could have the following
reasons:
The document is blocked by another user or another job.
There is a shipping due date index for the document although the
document is no longer due for shipment.
For the index list, the document fulfills all of the chosen selection
criteria. However, there is no item that fulfills all of these
criteria.

System Response

After the failed attempt to deliver the document, the system continues
with the processing. If the document is blocked, the system makes &V2&
attempts to get the block for the object, according to your settings
(SHP_VL10_REPEAT_BLOCKED: Transaction VL10CUC -> RepBlocked).

How to fix this error?

Start the transaction or the job again and attempt to process the
document again.

Procedure for System Administrators

Start the transaction or the job again to ensure processing of the
document.
Run report RVV05IVB or ZKORVETV for this document. Documents which
incorrectly had a shipping due date index no longer appear in the
shipping due date index.
With the same selection criteria, switch from an index list to a
document list. Documents with items that do not fulfill all of the
selection criteria chosen are not displayed.

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.