Shopify refused to update this product's stock because the update would make it active at a second location, and one of those locations is a fulfillment service location. Shuttle stops the transfer.
Why it happens
Shopify does not let a product be stocked at more than one location if one of them is a fulfillment service location. That is a location run by a fulfillment app or service.
When a product is stocked at exactly one location in Shopify, Shuttle updates the stock there, whatever your default is. When the product is stocked at several locations that do not include your Default Shopify location, or is not stocked anywhere yet, Shuttle uses your Default Shopify location. If that adds a location, and either the product's existing location or your default is a fulfillment service location, Shopify refuses.
Check these in Shopify:
Was your Default Shopify location added by a fulfillment app or service?
Is the product stocked at several locations, one of them a fulfillment service location, with your default not among them?
How to fix it
In Shuttle, go to Settings → Products → General.
Under Shopify inventory location, check the Default Shopify location.
If it was added by a fulfillment app or service, choose an ordinary location. This applies to all your products, so pick the one where you keep most of your stock.
If your default is already an ordinary location, either set Default Shopify location to one of the locations the product is stocked at, or in Shopify stock the product at one location only.
Click Save, then run the transfer again.
Still seeing it?
Contact support if the error returns when your default is an ordinary location and is one of the product's stocked locations, or when the product is stocked at one location only. Tell us the product's name.