SOAP Call with c#

I am attempting to make a SOAP call in c# and I am receiving the following error.

System.InvalidCastException: Unable to cast COM object of type 'MSXML2.ServerXMLHTTPClass' to interface type 'MSXML2.IServerXMLHTTPRequest2'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{2E01311B-C322-4B0A-BD77-B90CFDC8DCE7}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)). at MSXML2.ServerXMLHTTPClass.open(String bstrMethod, String bstrUrl, Object varAsync, Object bstrUser, Object bstrPassword)

My code is something like this

                strSOAP.Append("<?xml version='1.0' encoding='utf-8'?>");
                strSOAP.Append("<SOAP:Envelope xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' ");
                strSOAP.Append(" xmlns:xsd='http://www.w3.org/2001/XMLSchema' ");
                strSOAP.Append(" xmlns:SOAP='http://schemas.xmlsoap.org/soap/envelope/'> ");
                strSOAP.Append("<SOAP:Body>"  );
                strSOAP.Append("<m:my_method xmlns:m='http://mycompany.com#soapRegistrationServer'>");
... soap parameters ...
                  strSOAP.Append("</m:my_method></SOAP:Body></SOAP:Envelope>");

                 MSXML2.ServerXMLHTTP objHttp = new MSXML2.ServerXMLHTTPClass();
                 
                /// ERROR ON THIS LINE:
                 objHttp.open("POST", "https://mycompany/somefile.php", false, user, password);
                 
                 // send soap
                 objHttp.setRequestHeader("Content-Type", "text/xml");
                 objHttp.setRequestHeader("SOAPAction", "my_method");
                 objHttp.send(strSOAP.ToString());
                 
                 // print response
                 lblResponse.Text = objHttp.responseText;Start Free Trial