Shuttle could not sync a stock change because more than one Shopify product carries the same SKU as the Etsy listing, so it did not know which product to update.
Why it happens
When Etsy stock changes, Shuttle looks for the Shopify product it already matched. If that match is missing and Match products and variations by SKU is on, Shuttle searches Shopify for the listing's SKUs. When they turn up in two or more products, Shuttle stops instead of guessing.
Two or more Shopify products share a SKU with the Etsy listing. In Shopify, search your products for the SKU.
The matched Shopify product was deleted, and the products left with that SKU are copies of each other.
The product was never matched, and its SKU is reused across several Shopify products.
SKU matching is controlled under Settings > Products > General, in the Smart Matcher card.
How to fix it
Open Stock Syncer and find the failed sync in Sync History to see which product it is.
Go to Inventory > SKU Audit and turn on the Duplicates filter to see which products share SKUs. See SKU Audit: Detect Duplicates, Orphans, and Mismatches.
In Shopify, give each product and variant its own SKU, or delete the copy you no longer need. One SKU per product is enough when Etsy keeps stock at the product level.
To keep the duplicates, match the product yourself: go to Products, open the product's More actions menu, and choose Match to Shopify. See Matching Products & Variations.
Change the stock on Etsy again, or wait for the next change. Shuttle retries on every stock update.
Still seeing it?
If the SKUs are unique in Shopify, the product shows as matched on the Products page and the error still appears, contact support with the product title and SKU.