You specified a maximum weight as a capacity criterion.
In order to ensure that this criterion is met, all deliveries should be
sorted in descending order according to weight. This criterion is added
as the last sort criterion. This means that the sequence of the
deliveries, which you have already defined with different criteria,
remains unchanged (for example, sorting according to ship-to party).
When the system processes the capacity criteria, it considers the
deliveries of a shipment (according to their sequence). Each time a
delivery exceeds the maximum weight of the shipment, the system omits
this delivery and selects the next one.
In this way, the system can fill the shipments satisfactorily but not
optimally!

Error message extract from SAP system. Copyright SAP SE.