@Francis21 as mentioned above, this endpoint will ONLY return the reports (not the data, but the report definition) created using the same API access token. You are getting  because you didn't create any reports through the API.
@Francis21 only by leveraging Shopify Order API (REST or GraphQL) to go through the orders and pick the ones that have the extra information you are matching on (say UTM campaign) in the customAttributes. But this is all assuming the customAttributes are populated when the order is created on the store front (through a third-party integration or an app).
Hello I have encountered the same problem now, but I always fail to call the creation interface. I am using C# and the interface always returns: “The request could not be understood by the server due to malformed syntax”. I use the post method, The text passed is