c# System.Net.Http.HttpRequestException: An error occurred while sending the request

Question

System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
   at System.Net.Sockets.Socket.EndReceive(IAsyncResult asyncResult)
   at System.Net.Sockets.NetworkStream.EndRead(IAsyncResult asyncResult)
   --- End of inner exception stack trace ---
   at System.Net.TlsStream.EndWrite(IAsyncResult asyncResult)
   at System.Net.ConnectStream.WriteHeadersCallback(IAsyncResult ar)
   --- End of inner exception stack trace ---
   at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context)
   at System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar)
   --- End of inner exception stack trace ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at CanadianTireDashboard.Controllers.HomeController.<pickUpApi>d__109.MoveNext() in C:\Users\sbaqai\Documents\TFS\CanadianTireDashboard\CanadianTireDashboard\Controllers\HomeController.cs:line 865

I have given up. Idk why this works on our dev server but not on prod. Our dev and prod are on IIS. I am still new to C# and .NET Framework but after Googling around I still can't fix this error. Nothing works.

Here is my code that is trying to call HttpClient.PostAsync

private async Task pickUpApi(string billOfLading)
{
    ErrorFile f = new ErrorFile();

    HomeController hc = new HomeController();
    HttpResponseMessage pickUpRequestCall = null;
    
    //Send a request to url with an xml body
    try
    {

        //var url = "http://www.saiasecure.com/webservice/shipment/xml.aspx?";
        var url = "https://www.tst-cfexpress.com/xml/pickup";
        
        XmlSerializer xmlSerializer = new XmlSerializer(typeof(pickuprequest));
        XmlSerializerNamespaces ns = new XmlSerializerNamespaces();
        ns.Add("", "");
        pickuprequest cdn = new pickuprequest();

        var values = cdn.getAll(billOfLading);
        
        try
        {
            var path = $@"C:\CDNTireJobs\Test\{billOfLading}.xml";
            var encoding = Encoding.GetEncoding("ISO-8859-1");
            using (XmlTextWriter tw = new XmlTextWriter(path, encoding))
            {
                xmlSerializer.Serialize(tw, values, ns);
                tw.Close();
            }
        }
        catch (FileNotFoundException e)
        {
            f.errorFile($"File Exception {e.Message}");
        }

     
        var xmlcontent = System.IO.File.ReadAllText($@"C:\CDNTireJobs\Test\{billOfLading}.xml");
        var content = new StringContent(xmlcontent,
                        Encoding.GetEncoding("ISO-8859-1"), "application/xml");

        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12;
        ServicePointManager.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) => true;

        pickUpRequestCall = await httpClient.PostAsync(url, content);
        f.errorFile(pickUpRequestCall.StatusCode.ToString());
        if (pickUpRequestCall.StatusCode == HttpStatusCode.OK)
        {
            var strResBody = pickUpRequestCall.Content.ReadAsStringAsync().Result;
            f.errorFile(strResBody);
        }
    }
    catch(HttpRequestException e)
    {  
        f.errorFile(e.ToString());
    }
}

I tried using System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;