I’m having trouble with adding a reference to my project. I need the System.Web reference in order for my code to work but the reference won’t stick. When I look in the Solution Explorer, I found this alert icon on top of the reference I added. I have the same reference in another project within the same solution and it is working fine.

If you look at the properties of this reference, you’ll see that the Resolved attribute is set to False

Does anyone know how I can fix this or what’s causing this problem?
Good chance that your project is targetting .NET Framework 4.0 Client Profile. System.Web is not supported by the client profile. Change it to .NET Framework 4.0.