This error means Shuttle could not set stock levels in Shopify, because the inventory location Shuttle uses has been deactivated in Shopify. When this happens, Shuttle stops the transfer. You may also get an email from Shuttle about it.
Why it happens
The location you picked as Shuttle's default inventory location was deactivated in Shopify after you picked it. To check, look at your locations in your Shopify admin and see if that location is still active. In Shuttle, a deactivated location shows as "(Inactive)" in the location list after you refresh it.
How to fix it
In Shuttle, go to Settings → Products → General.
Find the Shopify inventory location section.
Click Refresh locations to get your latest locations from Shopify. If your store now has only one active location, Shuttle selects it for you. In that case, go to step 6.
In Default Shopify location, choose an active location. You cannot select locations marked (Inactive).
Click Save.
Run the transfer again. For help, see Transfer products between Etsy and Shopify.
Want to keep the same location? Reactivate it in Shopify, then run the transfer again. You do not need to change anything in Shuttle.
Still seeing it?
You picked an active location, saved, and the error still comes back? Contact Shuttle support. Tell us the name of the location you picked and when the transfer ran.