As of now, System got designed in such a way that, once an Order got PACKED and INVOICED system won't allow the user to cancel the Order. If you user still want to make the invoiced inventory LIVE, they only have an option to re-GRN the item.
Most of the channels won't allow partial cancellation(For B2C Orders). A B2C Order can only marked as COMPLETED once all items got packed and invoiced. Else if none of the picked items are PACKED and INVOICED, user could mark the Order as CANCELLED instead of marking it as COMPLETE.