Shuttle stopped this transfer or update because it could not use the Etsy listing linked to the product.
Why it happens
The listing was deleted on Etsy, or Shuttle could not load it. This can happen in both directions.
Etsy shows the listing as unavailable, or it is in a different Etsy shop from the one Shuttle is using. This happens when updating from Shopify to Etsy.
Your variation settings remove every variation. Your exclusions may match every variation, or every variation may be hidden on Etsy while "Only export visible variations" is on. Retrying will not help until you change them.
How to fix it
In Shuttle, go to Products, find the product, open its ... menu (More actions) and click View on Etsy. Check that the listing exists, is active and is in the right shop.
If the listing was deleted and the product is also on Shopify, open the ... menu and click Match to Etsy. Shuttle removes the old link and shows "No matching product found". Click Manually Match Product, or transfer the product to Etsy again. A product that only exists on Etsy has nothing left to transfer.
If the listing is in a different Etsy shop, open Match to Etsy, click Unmatch the product, then Yes, reset match. The window closes and shows "Product match was reset". Open Match to Etsy again. Shuttle then looks for a listing in the current shop with the same SKUs. Check that it is the right one. If none is found, click Manually Match Product or transfer the product to Etsy again.
Go to Settings > Products > Etsy → Shopify, find Product Variations and check Exclude variations by name and Only export visible variations. Leave at least one variation of the listing. See Excluding Specific Variations when Transferring Products to Shopify.
Run the transfer or update again.
Still seeing it?
Contact support if the listing exists, is active, is in the right shop, keeps at least one variation, and the error remains. Send the product name and the Etsy listing link.