"Unknown product" warning on seller central

There are two circumstances, where this message will show on the STA shipments page.

1) The listing is inactive or FBM

2) The SKU used to create the shipment on Seller Chain does not have a corresponding SKU on Seller Central.

You can confirm the reason by clicking the listing name hyperlink on that listing. If the link leads back to a listing the cause is reason 1. If there is no listing the reason for the message is cause 2. 

Cause 1 can be fixed by converting the listing to FBA, second cause can be fixed by creating a new listing or recreating the shipment with the correct SKU (SKUs are case and space-sensitive).