What causes this issue?

You have selected deliveries to execute a delivery split. Shipment &V1&
, which contains one or several deliveries, also contains means of
transport and packaging materials that were changed during the current
processing session. For deliveries in these types of shipments, the
split can only be executed after the shipments have been saved.

System Response

The current process is terminated by the system.

How to fix this error?

Save shipments that are currently being processed. Then select these
shipments again and execute the split from the start.

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.