How can I resolve the 'api_client not associated with fulfillment service' error?

[MAIharey]

We are trying migrate our Order API, to Fulfillment Order API, and we are calling this URL, with the code below, but we are receiving error as  The api_client is not associated with any fulfillment service

Please let us know how to resolve this error, we have around 30 sites to migrate

https://xxx.myshopify.com/admin/api/2023-01/assigned_fulfillment_orders.json?status=open&location_id...

var client = new RestClient();

client.Authenticator = new HttpBasicAuthenticator(apiKey, apiPwd);

var requestres = new RestRequest(shopifyURL, Method.GET);
requestres.AddHeader("X-Shopify-Access-Token", "xxxxxxxx");
requestres.AddHeader("header", "Content-Type: application/json");
IRestResponse response = client.Execute(requestres);

[MonicaG10]

Did you ever figure out what you needed to do to resolve this?