Is there a way to debug a standard API call?

Since the objects are hidden i can't find a way to debug them. I copied the entire salesOrder API to replicate it but i get different results passing the same Json. A "you must specify a document id to get the lines." Error on the custom API but it works on the standard API

{
    "customerNumber": "10000",
    "salesOrderLines":[{
        "itemId": "291b546c-1473-ec11-bb83-000d3aa85d68",
        "quantity": 5
        },
        {
        "itemId": "2d1b546c-1473-ec11-bb83-000d3aa85d68",
        "quantity": 3
        }]
}