This error means Shopify saved the product but did not send its variants back to Shuttle, so Shuttle could not link them.
Why it happens
Shopify had a short-lived problem while it saved the product. It created the product, but its reply to Shuttle came back without the product's variants. This passes on its own.
You see the error in the result of the transfer or update, and in the product's Transfer history, which is in the row's More actions menu (…). Depending on what Shopify sent back, Shuttle may not have linked the product at all, so its row shows either Update in Shopify or Transfer to Shopify.
How to fix it
Wait a few minutes.
In Shuttle, go to Products and open the Etsy → Shopify tab.
Find the product and check which button its row shows.
If it shows Update in Shopify, click it. In the Update product window, under Advanced, tick Recreate Shopify inventory. This rebuilds the product's variants in Shopify. Without it, the update skips the variants that are missing. Then click Update Shopify product.
If it shows Transfer to Shopify, first look for the product in your Shopify store. If it is there, open the row's More actions menu (…), choose Match to Shopify and pick that product, then follow step 4. If it is not there, transfer the product again.
If the error came from updating a product that was already matched and its row still shows Update in Shopify, you can skip ticking Recreate Shopify inventory and run the same update again.
Still seeing it?
If the error comes back after you retry with Recreate Shopify inventory ticked, contact our support team and tell us the product's name so we can look into it.