Etsy refused to let Shuttle work with your Etsy shop, so the product was not transferred.
Why it happens
Your Etsy shop is on vacation. Etsy does not allow updating products to a shop on vacation mode.
Etsy no longer accepts Shuttle's connection to your shop. This mostly affects shops connected to Shuttle a long time ago. To check, open Overview in Shuttle and look for a banner that says Etsy credentials need refreshing, Etsy credentials are no longer valid or Reconnect your Etsy shop.
How to fix it
If you shop is on vacation mode, turn it off, make the updates, and turn it back on again. Otherwise -
Open Overview in Shuttle.
If you see one of the banners above, click its button (Refresh credentials or Reconnect Etsy) and approve access on Etsy.
You can also go to Settings > Account > Connected Shops. If your shop shows Reconnect needed, click Reconnect, then Connect to Etsy, and approve access on Etsy.
Transfer the product again.
Still seeing it?
Contact our support team if your Etsy shop is not on vacation mode, you have reconnected it, and the next transfer still shows this error.