Nicht erkannte Konfiguration Abschnitt system.web/Konfiguration

ich bin immer das "nicht erkannte Konfiguration Abschnitt system.web/Konfiguration, wenn ich mein admin -, Mitarbeiter-und Benutzer-Authentifizierung .bitte helfen Sie mir mit this.by hier biete ich die web.config code

<?xml version="1.0"?>
<!-- 
    Note: As an alternative to hand editing this file you can use the 
    web admin tool to configure settings for your application. Use
    the Website->Asp.Net Configuration option in Visual Studio.
    A full list of settings and comments can be found in 
    machine.config.comments usually located in 
    \Windows\Microsoft.Net\Framework\v2.x\Config 
-->

<configuration>
<appSettings/>
<connectionStrings>
    <add name="ConnectionString" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\ASPNETDB.MDF;Integrated Security=True;User Instance=True" providerName="System.Data.SqlClient"/>
    <add name="ASPNETDBConnectionString" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=C:\Users\Se7en\Desktop\Personal\VIVA\1\App_Data\ASPNETDB.MDF;Integrated Security=True;Connect Timeout=30;User Instance=True" providerName="System.Data.SqlClient"/>
</connectionStrings>
<system.web>
    <!-- 
        Set compilation debug="true" to insert debugging 
        symbols into the compiled page. Because this 
        affects performance, set this value to true only 
        during development.

        Visual Basic options:
        Set strict="true" to disallow all data type conversions 
        where data loss can occur. 
        Set explicit="true" to force declaration of all variables.
    -->

<configuration>

  <system.web>

    <authentication mode="Forms">
      <forms loginUrl="Login.aspx">
        <credentials passwordFormat="Clear">
          <user name="admin" password="password"/>
        </credentials>
      </forms>
    </authentication>
    <authorization>
      <allow users="*"/>
    </authorization>
    <compilation debug="true"/>
  </system.web>
  <location path="adminstrator">
    <system.web>
      <authorization>
        <allow users="adminstrator"/>
        <deny users="*"/>
      </authorization>
    </system.web>
  </location>
  <location path="staff">
    <system.web>
      <authorization>
        <allow users="staff"/>
        <deny users="*"/>
        <allow roles="adminstrator" />
      </authorization>
    </system.web>
  </location>
  <location path="user">
    <system.web>
      <authorization>
        <deny users="?"/>
      </authorization>
    </system.web>
  </location>
</configuration>
<!-- 
<authentication mode="Forms"/>
-->
    <roleManager enabled="true"/>
    <compilation debug="true" strict="false" explicit="true" targetFramework="4.0">
        <assemblies>
            <add assembly="System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/></assemblies></compilation>
    <pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID">
        <namespaces>
            <clear/>
            <add namespace="System"/>
            <add namespace="System.Collections"/>
            <add namespace="System.Collections.Generic"/>
            <add namespace="System.Collections.Specialized"/>
            <add namespace="System.Configuration"/>
            <add namespace="System.Text"/>
            <add namespace="System.Text.RegularExpressions"/>
            <add namespace="System.Linq"/>
            <add namespace="System.Xml.Linq"/>
            <add namespace="System.Web"/>
            <add namespace="System.Web.Caching"/>
            <add namespace="System.Web.SessionState"/>
            <add namespace="System.Web.Security"/>
            <add namespace="System.Web.Profile"/>
            <add namespace="System.Web.UI"/>
            <add namespace="System.Web.UI.WebControls"/>
            <add namespace="System.Web.UI.WebControls.WebParts"/>
            <add namespace="System.Web.UI.HtmlControls"/>
        </namespaces>
    </pages>
    <!--
        The <authentication> section enables configuration 
        of the security authentication mode used by 
        ASP.NET to identify an incoming user. 
    -->
    <!--
        The <customErrors> section enables configuration 
        of what to do if/when an unhandled error occurs 
        during the execution of a request. Specifically, 
        it enables developers to configure html error pages 
        to be displayed in place of a error stack trace.

    <customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
        <error statusCode="403" redirect="NoAccess.htm" />
        <error statusCode="404" redirect="FileNotFound.htm" />
    </customErrors>
    -->
</system.web>

   <!-- 
    The system.webServer section is required for running ASP.NET AJAX under Internet
    Information Services 7.0.  It is not necessary for previous version of IIS.
   -->

</configuration>