It will not be possible to complete this shipment. There is a restriction which means that if one have started to attach lines to HU on a shipment, all lines needs to be attached. Then there an option to deliver shipment without attaching any of the shipment lines to HU. To use that option one should use set the option to “Unattached Shipment Lines Allowed”. This value can be dfaulted on the basic data for Customer / Address / Order Address Info

Here is help text for “Unattached Shipment Lines Allowed” checbox on shipment header which more or less gives the same answer as above:

“Unattached Shipment Lines Allowed
If this check box is selected, the shipment can be set to status Complete when having a handling unit structure defined without having to connect any shipment lines to the structure. However, if any shipment line has been connected to the structure, the system will require that all shipment lines that are connected to the shipment are connected to the structure as well. By default, the check box is not selected, which means that when a handling unit structure is defined, it should contain the exact contents of each handling unit in the handling unit structure with reference to the shipment line that is included. Without the shipment lines attached the status cannot be set to Complete for the shipment. When creating a new shipment, the value will be retrieved to the shipment from the applicable Receiver ID and address stated in the shipment. The value can be updated until the shipment reaches status Complete.”

As an option for your example, still wanting have some shipment lines packed and some not could perhaps be to use reassign functionality to create new shipment for parts that shouldn’t be packed. Or create a virtual HU for unpacked shipment lines.