This error means Etsy did not accept the product's variations, so they were not saved on Etsy.
Why it happens
Etsy needs every pairing of a product's options to exist. Take a product with Size and Color. If your Shopify product has Small in Blue but no Small in Red, Etsy still expects a Small in Red.
Shuttle normally adds the missing pairings for you as hidden options that can't be bought. You don't need to create them yourself. You see this error only when Shuttle could not add them for this product. That is a problem on Shuttle's side, not something you set up wrong.
You can only get this error on products with two or more options that Shuttle sends from Shopify to Etsy.
What it means for your listing
On a first transfer, the listing is created on Etsy as a draft without its variations. Buyers can't see it. Look for it in your Etsy drafts.
On an update, the listing's variations, prices, quantities and SKUs on Etsy stay as they were.
How to fix it
In Etsy, open your drafts and check that the listing is there.
Update the product from Shuttle once more. See Updating products with Shuttle.
Under "What to Update?", tick "Overwrite All", or tick "SKU", "Price" and "Quantity" together. Then tick "Recreate Etsy Inventory". An update with only some of these fields leaves the listing without variations. See Update Product Variations with "Recreate Inventory".
If the update works, the variations appear on Etsy and the error clears.
Still seeing it?
If the same error comes back after you try again with "Recreate Etsy Inventory" ticked, contact support. Tell us the product's name. Our team needs to fix this one. Changing your settings won't clear it.