HTTP Error 500.19 - Internal Server Error The requested page cannot be accessed because the related con how to solve this error?

Error Summary
HTTP Error 500.19 - Internal Server Error
The requested page cannot be accessed because the related configuration data for the page is invalid.
Detailed Error Information
Module  IIS Web Core
Notification    BeginRequest
Handler Not yet determined
Error Code  0x80070021
Config Error    This configuration section cannot be used at this path. This happens when the section is locked at a parent level. Locking is either by default (overrideModeDefault="Deny"), or set explicitly by a location tag with overrideMode="Deny" or the legacy allowOverride="false".
Config File \\?\C:\SchoolDemo\SchoolDemo\SchoolDemo\web.config
Requested URL   http://localhost:80/Demo/Demo.aspx
Physical Path   C:\SchoolDemo\SchoolDemo\SchoolDemo\Demo.aspx
Logon Method    Not yet determined
Logon User  Not yet determined
Config Source
   49:   <system.webServer>
   50:      <modules runAllManagedModulesForAllRequests="true"/>
   51:   </system.webServer>
Links and More Information
This error occurs when there is a problem reading the configuration file for the Web server or Web application. In some cases, the event logs may contain more information about what caused this error.
View more information »