Visual Studio 2010 : Can't change target. Gives TargetFrameworkMoniker Error

This happened to me because my project file was marked as "Read Only" in its properties. Changed it, and it was fine.

It also could be caused by read-only app.config/web.config

Found the solution. Simply opened app.config in text pad (it was empty for some reason) and pasted:

<?xml version="1.0"?>
<configuration>
<startup><supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/></startup></configuration>