In which section you have added these elements
It has to be like this
<?xml version="1.0"?>
<!--
For more information on how to configure your ASP.NET application, please visit
http://go.microsoft.com/fwlink/?LinkId=169433
-->
<configuration>
<system.web>
<httpRuntime executionTimeout="240" maxRequestLength="20480" />
<compilation debug="true" targetFramework="4.0"/>
</system.web>
</configuration>