What causes this issue?

The total number of deliveries in the shipment created by the system is
lower than the defined minimum number of deliveries per shipment.
This means that the shipment document was not created and the delivery
has not been assigned to a shipment.

System Response

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?

Error message extract from SAP system. Copyright SAP SE.