Saving not possible because external shipment numbers missing

What causes this issue?

You have attempted to save one or several shipments that require the
assignment of an external shipment number because of the number range
setting in the shipment type. However, external shipment numbers have
not been assigned for all the shipments to be saved. There are still
shipments with temporary numbers ($0001, $0002,...).

System Response

The save process is cancelled.

How to fix this error?

On the overview screen, proceed to the "Identification" section and
enter the external shipment number there. Then start the save process
once 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.