You aren’t querying Shopify API there but the Flow environment and the Flow environment doesn’t support fields that require arguments like quantities. We haven’t build the solution yet for fields other than metafield and available.
Only workaround would be to write a task that does it via a Flow extension in an app. When Send HTTP request can return data, you could use that to query the API and get the values
You might be able to solve your use case using the Send HTTP Request action provided by the Flow Companion app. This action returns request data, which you can use in subsequent steps.
Would prefer to do it without an extra app.
This is one edge case where product data stored in Shopify, and accessible through the API, is not reachable by Flow.
Thanks.