Error: 'System.NotSupportedException' for SecurityProtocol on IIS

hi,

i have configured paypal on my server and added these lines to enabl tls requested by paypal in my application

but when i open the page it gives me following error

https://snag.gy/fh587e.jpg

what should i do  Please advice

  ServicePointManager.Expect100Continue = true;
        ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;
        ServicePointManager.DefaultConnectionLimit = 9999;

        //end of server protocal

        //Post back to either sandbox or live
        string strSandbox = "https://www.sandbox.paypal.com/cgi-bin/webscr";


        //post of live
        string strLive = "https://www.paypal.com/cgi-bin/webscr";



        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(strSandbox);
        //Set values for the request back
        req.Method = "POST";
        req.ContentType = "application/x-www-form-urlencoded";
        byte[] param = Request.BinaryRead(HttpContext.Current.Request.ContentLength);
        string strRequest = Encoding.ASCII.GetString(param);
        string strResponse_copy = strRequest;  //Save a copy of the initial info sent by PayPal
        strRequest += "&cmd=_notify-validate";
        req.ContentLength = strRequest.Length;

        //for proxy
        //WebProxy proxy = new WebProxy(new Uri("http://url:port#"));
        //req.Proxy = proxy;
        //Send the request to PayPal and get the response
        StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
        streamOut.Write(strRequest);
        streamOut.Close();
        StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
        string strResponse = streamIn.ReadToEnd();
        streamIn.Close();

        if (strResponse == "VERIFIED")
        {
}