Etsy turned down a stock update for this product because one group of variations was given two different quantities.
Why it happens
On this Etsy listing, the quantity varies by only some of the options, not all of them. For example, the quantity may vary by size but not by color. All variations that share that option then share one stock number on Etsy. If a stock update gives those variations different numbers, Etsy refuses the whole update.
Shuttle now gives each group a single quantity before it sends a stock update, so this error should be rare. When it does happen, Shuttle tries this product again the next time its stock changes.
How to fix it
In Shuttle, open Stock Syncer and find the product in the sync history.
If a newer sync for the same product succeeded, you don't need to do anything.
If the product keeps failing, open the listing in Etsy and edit its variations. Check which options the quantity varies by.
If each variation should have its own stock, set the quantity to vary by every option. Then enter the correct quantity for each variation.
Change the product's stock in Shopify, or wait for its next stock change, so Shuttle tries again.
For more on how Etsy tracks stock for a whole product or for each option, see Handling Etsy Stock Management by Product or Option Level.
Still seeing it?
If the same product still shows this error after you have checked its quantity setup on Etsy and its stock has changed again, contact support. Include the product's name so we can look at that listing.