Shuttle saw a stock change on an Etsy variation but could not find the Shopify variation that goes with it. That variation's stock was not updated in Shopify.
Why it happens
Shuttle pairs each Etsy variation with a variation in the matched Shopify product. It keeps any pairing it already has. To make a new pairing, it first looks for a Shopify variation with the same option values, then for one with the same SKU. You see this error when neither check finds a match:
The variation exists only on Etsy. You added it on Etsy but not to the Shopify product.
Both the option values and the SKUs are different. For example, the variation is "Navy" on Etsy and "Dark Blue" in Shopify, and the SKUs are blank or not the same.
The option values are different and the SKU is shared. Another variation in the same product uses that SKU, so Shuttle cannot use the SKU to pair them.
How to fix it
Open the product in Etsy and in Shopify, and find the variation whose stock changed.
Check that the Shopify product has that variation. If it is missing, add it.
Give the two variations the same option values or the same SKU. Capital letters and the order of the options do not matter. A SKU only pairs them when no other variation in the product uses it. If the SKU is shared, see Duplicate SKUs Within a Product.
You do not need to do anything else. Shuttle checks the pairing again on every stock change on Etsy, so the next change updates the variation.
Still seeing it?
Contact support if the error comes back after a new stock change on Etsy, even though the option values or SKUs match. Tell us the product name and which variation it is.