There are 2 ways to remove this warning
First, you may need to convert the current .Net Framework used by the application to higher version, let’s say from .net framework 2 to 3.5
- Goto Property Pages of the web site project
- View Menu->Property Pages or press shift+F4
- Click the Build option from the left side
- Make sure the Target Framework is .Net Framework 3.5
Second, remove the namespace that shown in the warning in this case the “System.Xml.Linq” from the web.config
