Error message : the request was aborted: could not create SSL/TLS secure channel.

Exception Read: The request was aborted: Could not create SSL/TLS secure channel.

//On the top of the page
using System.Net;
using System.IO;

//My methods
        private void btnGetData_Click(object sender, EventArgs e)
        {

            StringBuilder sb = new StringBuilder();

            try
            {

                // used on each read operation
                byte[] buf = new byte[8192];

                ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(AcceptAllCertifications);
                Uri uri = new Uri("https://www.zillow.com/homedetails/2138325511_zpid");
                WebRequest webRequest = WebRequest.Create(uri);
                WebResponse webResponse = webRequest.GetResponse();
                //ReadFrom(webResponse.GetResponseStream());
                Stream response = webResponse.GetResponseStream();


                string tempString = null;
                int count = 0;
                do
                {
                    // fill the buffer with data
                    count = response.Read(buf, 0, buf.Length);

                    // make sure we read some data
                    if (count != 0)
                    {
                        // translate from bytes to ASCII text
                        tempString = Encoding.ASCII.GetString(buf, 0, count);

                        // continue building the string
                        sb.Append(tempString);
                    }
                }
                while (count > 0); // any more data to read?
            }
            catch (Exception ex2)
            {
                try
                {

                    StreamWriter sw = new StreamWriter(Environment.CurrentDirectory + "\\Error.txt");

                    //Write a line of text
                    sw.WriteLine("Exception Read: " + ex2.Message);

                    //Close the file
                    sw.Close();
                    this.Close();
                    return;
                }
                catch(Exception exinner2)
                {
                    Console.WriteLine("Exception Read: " + exinner2.Message);
                    this.Close();
                    return;
                }
            }

            // write to text file page source
            try
            {

                //Pass the filepath and filename to the StreamWriter Constructor
                StreamWriter sw = new StreamWriter(Environment.CurrentDirectory + "\\Pageoutput.txt");

                //Write a line of text
                sw.WriteLine(sb.ToString());

                //Close the file
                sw.Close();
                MessageBox.Show("Data inserted Successfully");
            }
            catch (Exception ex)
            {
                try
                {

                    StreamWriter sw = new StreamWriter(Environment.CurrentDirectory + "\\Error.txt");

                    //Write a line of text
                    sw.WriteLine("Exception Write: " + ex.Message);

                    //Close the file
                    sw.Close();
                }
                catch (Exception exinner)
                {
                    Console.WriteLine("Exception Write: " + exinner.Message);
                }
            }

        }

        public bool AcceptAllCertifications(object sender, System.Security.Cryptography.X509Certificates.X509Certificate certification, System.Security.Cryptography.X509Certificates.X509Chain chain, System.Net.Security.SslPolicyErrors sslPolicyErrors)
        {
            return true;
        }