unrecognized configuration section 'connectionStrings'

If you get the error like unrecognized configuration section 'connectionStrings' in ASP.NET application then the reason behind is your application is not running under the ASP>NET version 2.0. It is running under the ASP.NET version 1.1.4322.
To change this, please follow the steps below.

1. Go to IIS and select the folder Website
   
2. Select Default Websites
   
3. Right click on the default web sites
   
4. Select properties and choose Asp.net tab
5. Here select the ASP.NET version to 2.0

Note: To allow ASP.NET version 2.0 you need to do this. In IIS, Web service Extensions allow Asp.net v2.0.50727.