Etsy turned down an update to this listing because two variations that must share one SKU were sent with different SKUs.
Why it happens
On some listings, Etsy keeps a single SKU for a whole group of variations. For example, it keeps one SKU for every Colour of the same Size. If an update gives two variations in the same group different SKUs, Etsy refuses the whole update.
Shuttle now checks your SKUs before it sends an update. If they differ inside a group, Shuttle lets each variation keep its own SKU. If the listing shares its stock between variations, Shuttle keeps the SKUs that are already on Etsy. When that happens, you see the notice explained in SKUs were left as they are because this listing shares its stock between variations.
In most cases the next update goes through without you doing anything.
How to fix it
Find the failed update. For a stock sync, open Stock Syncer in Shuttle and look in Sync History. For a transfer or a product update, open Products and select History.
Open Products, select the Shopify → Etsy tab and find the product.
On its row, select Update in Etsy.
Tick SKU, Price or Quantity, or tick Select all fields. Only these fields update the listing's variations, which is where this error comes from. Then select Update Etsy product.
Wait for the window to show the result. You can also check it later in Products, under History.
Still seeing it?
If the same product fails with this error again after a new update, contact support. Tell them which product it is and whether a transfer, an update or a stock sync failed.