Apple Pay c#. Response status code does not indicate success: 417 (Expectation Failed)

On step onvalidatemerchant I call service that make response to apple and should returns "apple session", when I make request to Apple apple enpoint returns 417 code.

I'm trying to get "merchant session" from Apple Pay Sandbox but my code always returns an error "417 (Expectation Failed)". I tried different ways with WebRequestHandler, httpclient, different headers but result is the same.

Example of my service logic:

static void Main(string[] args)
{
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
    ServicePointManager.Expect100Continue = false;
  
    using (var httpClient = GetSecureHttpClient())
    {
        var jsonData = "{\"merchantIdentifier\" : \"merchant.com.domain.www\", \"domainName\" : \"www.domain.com\",  \"displayName\" : \"MySite\" }";


        using (var content = new StringContent(jsonData, Encoding.UTF8, "application/json"))
        {
            try
            {
                httpClient.DefaultRequestHeaders.ExpectContinue = false;
                using (var response = httpClient.PostAsync("https://apple-pay-gateway-cert.apple.com/paymentservices/startSession", content).Result)
                {
                    response.EnsureSuccessStatusCode();
                    Console.WriteLine(response.Content.ReadAsStringAsync().Result);
                }
            }
            catch (Exception ex) { Console.WriteLine("Apple Pay Gateway error: " + ex.ToString()); }
        }
    }
}


public static HttpClient GetSecureHttpClient()
{
    var handler = new WebRequestHandler();
    handler.ClientCertificates.Add(new X509Certificate2(("C:\\merchantid_certificate.p12"), "", X509KeyStorageFlags.MachineKeySet)); 
    return new HttpClient(handler, true);
}