This error means Shopify refused to create or update a product's variants because the inventory location Shuttle uses no longer exists in your Shopify store.
Why it happens
Shuttle puts stock for new and updated Shopify variants in one inventory location, the one picked under Default Shopify location in your settings. If that location was deleted in Shopify, Shopify rejects the variants and the product is not transferred or updated.
To check: in Shopify, go to Settings → Locations and see if the location you picked in Shuttle is still listed.
How to fix it
In Shuttle, go to Settings → Products → General.
Scroll to the Shopify inventory location section.
Click Refresh locations. Shuttle then gets the latest list of locations from Shopify.
If you have more than one location, open the Default Shopify location dropdown and choose a location that still exists. Locations marked (Inactive) cannot be selected.
Click Save.
Transfer or update the product again.
If your store has only one active location, Shuttle selects it for you when you click Refresh locations. You only need to run the transfer or update again.
Still seeing it?
If you get the same error after you refresh, pick a location that exists and run the transfer again, contact our support team. Tell us the product's name and the location you selected.