HTTP Error 500.19 - Internal Server Error error code is 080070021 how to solve this?

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>