Leg &1: Delivery &2 cannot be assigned because it is not in the shipment

What causes this issue?

The system has tried to assign a delivery to a leg but the delivery is
not in this shipment.

System Response

The system issues an error message and stops processing.

How to fix this error?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.