Shuttle could not load the Shopify product it needed to transfer or update this product on Etsy. It stopped before changing anything on Etsy for this product.
Why it happens
The product was deleted in Shopify. Search for it in your Shopify admin. If it is not there, this is the cause.
Shopify did not return the product at that moment. If the product is still in your Shopify admin, the problem was brief.
How to fix it
If the product is still in Shopify:
In Shuttle, go to Products and find the product.
Run it again with Transfer to Etsy or Update in Etsy.
If you deleted the product and no longer sell it, there is nothing to transfer. You can ignore this error.
If you deleted the product and created it again in Shopify, connect the Etsy listing to the new product:
In Shuttle, go to Products and find the Etsy listing.
Open the row's More actions menu and choose Match to Shopify.
If Shuttle shows the wrong product, choose Click here to un-match, then Yes, reset match. Then open Match to Shopify again.
If Shuttle finds no match, choose Manually Match Product. Search for the new Shopify product by name, Shopify ID or SKU, and select it.
Choose Match to product. If that Shopify product is already matched to another listing, the button says Unmatch and match here.
Run Update in Etsy again.
For more on matching, see Matching Products & Variations.
Still seeing it?
Contact support if the product is in your Shopify admin and the error comes back every time you run it again.