Shuttle could not read how many of this product you have at your chosen Shopify inventory location. It skipped the Etsy stock update, so the Etsy quantity has not changed.
You can only see this on Etsy listings that have one quantity for the whole listing. Listings with a separate quantity for each variation do not show it.
Why it happens
The variant is not stocked at your chosen location. It is stocked at other Shopify locations, and Shuttle cannot tell which of them to read from.
The variant is not stocked at any location in Shopify.
Inventory tracking is off in Shopify for the variants linked to this Etsy listing.
How to fix it
In Shuttle, go to Settings → Products → General.
Under Shopify inventory location, check that Default Shopify location is where you keep stock for the products you sell on Etsy. If a location is missing from the list, click Refresh locations.
Click Save.
In Shopify, check that every variant of the product is stocked at that location and has inventory tracking turned on.
Update the product on Etsy again, or wait for its next stock change in Shopify.
Do you keep stock for the same product at several locations on purpose? Then on the same page, uncheck "My Shopify variations are stocked in a single location". Next, set Quantity source for Etsy updates to "Use combined quantity across all locations (use with caution)" and click Save. Shuttle then sends Etsy your total across all locations. If an Etsy sale is bigger than the stock at your selected location, that location can go below zero.
Still seeing it?
Contact support if the error comes back after the next update, even though your location is correct and the variant is stocked there with tracking on. Tell us the product name.