This error means Etsy refused the product because Shuttle sent it in a language your Etsy shop can't accept.
Why it happens
Shuttle sends every product to Etsy in the language picked under Transfer to Etsy in. Etsy rejects the product when:
The language isn't your Etsy shop's own language. Etsy only lets new listings be created in the language your shop is set up in. Other languages can only be added as translations.
The language is no longer turned on in your Etsy shop. For example, it was removed in Etsy after you picked it in Shuttle.
How to fix it
In Shuttle, go to Settings > Products > General.
Scroll to Shop languages.
Under Transfer to Etsy in, pick the language your Etsy shop is set up in.
Click Save.
Run the transfer again.
The list only shows languages that are turned on in both your Etsy shop and your Shopify store. If your Etsy shop's language is missing, add that language to your Shopify store, then reload the settings page.
If you also want your products in other languages on Etsy, see Syncing Product Translations Between Etsy and Shopify.
Still seeing it?
Contact support if the error is still there after you saved your Etsy shop's language and ran the transfer again. Support can also help if no languages show under Shop languages.