I am getting the error "the remote server returned an error: (500) internal server error"

   using (var client = new WebClient())
            {
                // read the raw SOAP request message from a file
                string data = @"<soapenv:Envelope xmlns:soapenv=""http://schemas.xmlsoap.org/soap/envelope/"" xmlns:urn=""urn:enterprise.soap.sforce.com""><soapenv:Header><urn:LoginScopeHeader><urn:organizationId>organizationid</urn:organizationId>
       <!--Optional:-->
         <urn:portalId></urn:portalId>
      </urn:LoginScopeHeader>
   </soapenv:Header>
   <soapenv:Body>
      <urn:login>
         <urn:username>username</urn:username>
         <urn:password>password</urn:password>
      </urn:login>
   </soapenv:Body>
</soapenv:Envelope>";

                //client.Headers.Add("Content-Type", "application/soap+xml;  charset=utf-8");
                // The SOAPAction header indicates which method you would like to invoke
                // and could be seen in the WSDL: <soap:operation soapAction="..." /> element
                // client.Headers.Add("SOAPAction", "http://ws.vantiv.com/Shared/CardPayment/v2/AuthorizeKeyedCardPayment");
                try
                {
                    var response = client.UploadString("https://test.salesforce.com/services/Soap/c/40.0/0DF4B00000000as", data);
                     Util.ErrorWriting(DateTime.Now + " :::: " + MethodBase.GetCurrentMethod() + " :::: " + response);

                }
                catch (Exception ex)
                {
                    // return ex.Message;
                    Util.ErrorWriting(DateTime.Now + " :::: " + MethodBase.GetCurrentMethod() + " :::: " + ex.Message);
                }
            }