A page can only have one server-side form tag.
Masterpage.aspx
<html xmlns="http://www.w3.org/1999/xhtml"> <head runat="server"> <title></title> <asp:ContentPlaceHolder ID="head" runat="server"> </asp:ContentPlaceHolder> <script src='http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js'> </script> <script src="Chrome.js" type="text/javascript"></script> </head> <body> <form id="Master" runat="server"> <table style="width:987px; margin:auto; height: 86px;"> <tr style="background-color:#FFFFFF"> <td class="style2"> <img src="images/logo.jpg" alt="" width="235" height="73" /> </td> <td style="vertical-align:middle;"class="style1"> <div style="float:right; width:153px; height: 22px;"> <asp:TextBox ID="TextBox1" runat="server" Height="20px" style="margin-left: 0px" Width="140px"> </asp:TextBox> </div> <a href="">Forget Password?</a> </span></span> </td> <td class="style33"> <div style="float:right; width:157px; height: 20px; margin-right: 1px;"> <asp:TextBox ID="TextBox2" runat="server" Height="20px" style="margin-left: 0px; margin-top: Width="147px"></asp:TextBox> </div> <a href="Registration.aspx">New User Registration</a> </span> </td> <td class="style40"> <div style="height: 67px; width: 81px;">  <asp:Button ID="btnlogin" runat="server" style="color:White; background-color:#404040;font-family:Verdana;font-size:10pt; height: 29px; margin-top: 12px;" Text="Login" onclick="btnlogin_Click1" /></div> </td> </tr> </table> </td> </tr> <tr style="background-color:#207DA8"> <td class="style39"> </td> <td class="style39"> </td> </tr> <script type="text/javascript"> cssdropdown.startchrome("chromemenu") </script> </td> </tr> <img alt="" src="images/rgt.jpg" style="width: 471px; height: 247px" /> </td> </tr> <tr style="vertical-align=top;background-color=#D4D3D9"> <td style="height:1;"> </td> </tr> </tbody> </table> </tbody> </table> <asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server"> </asp:ContentPlaceHolder> </form> </body> </html> RegisterPage.aspx
<%@ Page Language="C#" MasterPageFile="~/MasterPage.Master" AutoEventWireup="true" CodeBehind="Registration.aspx.cs" ValidateRequest="false" Inherits="Paragraphreader.Registration" %> <asp:Content ContentPlaceHolderID="ContentPlaceHolder1" runat="server"> <script src="jquery.validate.js" type="text/javascript"></script> <script src="jquery.validation.js" type="text/javascript"></script> <script> $("#Register").submit(function () { return $(this).validation(); alert("submitted"); }); </script> <script> function validate() { var a = 0, rdbtn = document.getElementsByName("gender"); for (i = 0; i < rdbtn.length; i++) { if (rdbtn.item(i).checked == false) { a++; } } if (a == rdbtn.length) { document.getElementById("RadioButtonList1").style.border = "2px solid red"; return false; } else { document.getElementById("RadioButtonList1").style.border = ""; return true; } } </script> <link href="Career.css" rel="stylesheet" type="text/css" /> <link href="Chrome.css" rel="stylesheet" type="text/css" /> <link href="Styles.css" rel="stylesheet" type="text/css" /> <form id="Register" runat="server">//A page can have only one form tag <asp:ScriptManager ID="ScriptManager1" runat="server"> </asp:ScriptManager> <asp:UpdatePanel ID="FormUpdatePanel" runat="server"> <ContentTemplate> I get an error (there can be only one side on the server side of the page when compiling RegisterPage.aspx. On the main page, I have a form tag and server-side code On the "Register" page, I have a form tag and server-side code How to avoid this error.
Master pages should not contain form tags at all, as they are intended to be used only as the base layout of your content page. Remove the form tag from the main page and the error goes away.
If you want to define a common “Login” section for all of your pages (this is apparently your intention), you can create a UserControl instead. Then you define a section on your main page to contain this user control, making sure that the ContentPlaceHolder from the User UserControl always ends on the form content page. Thus, when the form is btnlogin_Click1 click event is btnlogin_Click1 .
In conclusion: reinstall your page so that you have one form tag per page, since ASP.NET does not allow you to have more than one and try to define your form inside content pages, not the Master page.