Moved to D365 Supply Chain Management forum.
You can't import directly in to the tables that contain posted transactions. These transactions are populated via x++ processes that do much more than only create records in these tables.
Also in D365 you can't go to Packing slip journal, click New and add new records manually.
The correct way to create packing slips is to post them from sales order, not write data to CustPackingSlipJour/CustPackingSlipTrans. Then the system also handles all inventory and financial related updates in other tables.
If you would like to do this via your own data entity, you should still not import directly into those tables. Instead you should import into some new helper table which contains fields such as InventTransId (of the sales line) and Qty to be posted to packing slip. Then you would develop x++ process that takes the data from this table and posts the packing slips. Or, develop a custom service instead of an entity, and let it handle the posting.