Help With fulfillments.json api returning 406

Retired Boards Appdev Fulfillment Api
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1

Hello, I am making a request to:

https://{redacted_shop}.myshopify.com/admin/api/2023-04/orders/5877504573716/fulfillments.json

{
    "fulfillment": {
        "line_items_by_fulfillment_order": [
            {
                "fulfillment_order_id": 6831985983764 
            }
        ],
        "tracking_info": {
            "number": "MS1562678",
            "url": "https://www.my-shipping-company.com?tracking_number=MS1562678"
        }

    } 
}

this is the request I make, with my accesstoken, accept and content-type as headers on postman.
This is what the api documents you should send to fulfill an order, however, I always get returned 406 not acceptable. Help would be very much appreciated