IIS 7.5 HTTP 500.19 Error is mission a section declaration

I have got an HTTP Error 500.19 since the related configuration data for the page is invalid.

Module: IIS Web Core
Notification: Unknown
Handler: Not yet determined
Error Code: 0x80070032
Config Error: The configuration section 'system.web.extensions' cannot be read because it is missing a section declaration: 

<system.web.extensions>
  <scripting>
    <webServices>
      <jsonSerialization maxJsonLength="2147483644" />
    </webServices>
  </scripting>
</system.web.extensions>

Sorry please could you explain me what is missing in my web.config file? Thank you!

the reason behind the issue is you’ve deployed your application on to an IIS Server where the application’s AppPool is set to run under the DefaultAppPool. DefaultAppPool is automatically configured to run under .NET 2.0 rather than .NET 4.0. This means that the System.Web.Extensions stuff is unavailable to that version of the .NET Framework.

you could try to follow the below steps to change the Application Pool (AppPool) settings for your site or application:

1)open iis select your site/application.

2)Under Actions, click Basic Settings…

3)Your application pool is probably set to DefaultAppPool.

4)Change the Application Pool to ASP.NET v4.0 (or another .NET 4.0 app pool) by clicking on the Select… button.

5) Click OK

recycle the app pool and restart the site.