Change the DownloadScript method with below and check.

public void DownloadScript()
{
    string iDValues = "", insertQry, updateQry;
    int i = 0;
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    SqlConnection con = new SqlConnection(conString);
    SqlCommand cmd = new SqlCommand("select * from Estate_TR", con);
    primaryKey = GetPrimaryKey(tableName, conString);
    insertQry = "";
    con.Open();

    if (File.Exists(Server.MapPath("~/" + tableName + ".sql")))
    {
        File.Delete(Server.MapPath("~/" + tableName + ".sql"));
    }

    // Writing Create Table Script.
    StreamWriter sw = new StreamWriter(Server.MapPath("~/" + tableName + ".sql"), true, Encoding.UTF8);
    sw.Write(GenerateCreateTableScript() + Environment.NewLine);
    sw.Close();

    SqlDataReader sdr = cmd.ExecuteReader();
    while (sdr.Read())
    {
        i = i + 1;
        updateAdd = "";
        insertQry = "";

        string celldata = "", coulmenName = "";
        for (int j = 0; j < sdr.FieldCount; j++)
        {
            if (j > 0)
            {
                coulmenName += "," + sdr.GetName(j).ToString();
                if (!string.IsNullOrEmpty(sdr[j].ToString()))
                {
                    celldata += ",N'" + sdr[j].ToString() + "'";
                }
                else
                {
                    celldata += ",'" + sdr[j].ToString() + "'";
                }
            }
            else
            {
                coulmenName += sdr.GetName(j).ToString();
                if (!string.IsNullOrEmpty(sdr[j].ToString()))
                {
                    celldata += "N'" + sdr[j].ToString() + "'";
                }
                else
                {
                    celldata += "'" + sdr[j].ToString() + "'";
                }                        
            }
            if (primaryKey == sdr.GetName(j).ToString())
            {
                iDValues = sdr[j].ToString();
            }
            if (iDValues != null)
            {
                updateQry = UpdateQuery(coulmenName, celldata, primaryKey, iDValues);
                updateAdd += updateQry;
                insertQry = InsertQuery(coulmenName, celldata, tableName);
            }
        }
        WriteScripts(tableName, insertQry, updateAdd, iDValues, primaryKey, i);
    }

    // Download the sql script file.
    if (File.Exists(Server.MapPath("~/" + tableName + ".sql")))
    {
        byte[] bytes = File.ReadAllBytes(Server.MapPath("~/" + tableName + ".sql"));
        File.Delete(Server.MapPath("~/" + tableName + ".sql"));

        Response.Clear();
        Response.Buffer = true;
        Response.Charset = "";
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        Response.ContentType = ContentType;
        Response.AppendHeader("Content-Disposition", "attachment; filename=" + tableName + ".sql");
        Response.BinaryWrite(bytes);
        Response.Flush();
        Response.End();
    }
}