This error means a stock change on Etsy could not be copied to the matched Shopify product, because Shopify sent back that product without any variants.
Why it happens
In Shopify, stock is stored on each variant. Every Shopify product has at least one variant, even if it has no options. When Shopify sends a product back with none, Shuttle checks again once. If the second answer is also empty, Shuttle skips that stock update. This usually happens when:
The product's options or variants were being changed in Shopify at the moment the stock changed on Etsy.
Shopify had a short-lived problem and sent back incomplete product details.
Shuttle does not try the skipped update again on its own. The next stock change for that listing is synced as normal.
How to fix it
In Shuttle, open the Stock Syncer page and find the product with this error in the sync history.
In your Shopify admin, go to Products and open that product.
Check that the product opens normally and shows its variants (or a single price and quantity if it has no options). If you were editing its options, finish and save your changes.
Sync the stock again. Change the quantity of the listing on Etsy, or use Bulk update quantities between Etsy and Shopify.
Check the Stock Syncer page again. The new update should finish without this error.
Still seeing it?
The product may look normal in Shopify and still get this error on every new stock change. In that case, contact our support team. Tell us the product name and roughly when the error appeared.