Etsy would not create the listing because it arrived without a shipping profile, and Shuttle had no default profile to add.
Why it happens
Every physical Etsy listing needs a shipping profile. When Shuttle transfers a product to Etsy it uses the profile matched to that product, or your Etsy default, or the first Etsy profile it knows about. This error means none of those existed, which comes down to one of these:
Shuttle has no record of your Etsy shipping profiles. The Shipping Profiles page shows a "No Etsy shipping profiles" notice. Either your Etsy shop has no profile yet, or Shuttle has not pulled your profiles in since you connected.
No Etsy default is selected. The "Etsy default" field on the Shipping Profiles page is empty.
Digital products are not affected. Etsy does not ask for a shipping profile on those.
How to fix it
In Shuttle, open Settings, then Catalog Mapping, then Shipping Profiles.
Under "Match shipping profiles", click Refresh so Shuttle pulls your current Etsy profiles.
If a "No Etsy shipping profiles" notice stays, create a shipping profile in Etsy and click Refresh again. If you want to start transferring right away, click "Create temporary profile" instead and replace it in Etsy later. Shuttle may first ask you to grant an extra Etsy permission.
Under "Default shipping profiles", pick a profile in the "Etsy default" field.
Click Save.
Transfer the product again.
If the Shipping Profiles page says your Shopify store moved to market-driven shipping, set the "Etsy default" on the Shipping → Collections page instead, then save and transfer again.
To match specific products to specific profiles, see Ensuring Products Are Assigned the Correct Shipping Profiles.
Still seeing it?
If the Etsy default is selected, your profiles appear after a refresh, and the transfer still fails, contact support with the product name and the Etsy shop it is going to.