No delivery proposal for FU &1: a delivery has already been created

What causes this issue?

Technical explanation:
Field DLV_CREATED_ON of the freight unit root data contains the
timestamp indicating when the system sent a delivery proposal for this
freight unit. Only an empty value is allowed in this context.

System Response

This freight unit is excluded from the creation of delivery proposals.

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.