Not all handling units could be assigned

What causes this issue?

There are either handling units or packing proposals that refer to more
than one delivery.

System Response

These handling units will not be created or assigned, since all
handling units must refer to only one delivery. In other words, all
handling units must be assigned to the same delivery.

How to fix this error?

Check to see if a delivery split occurred as the delivery was created.
Either change the delivery data or the pack data that led to the split.

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.