The type 'EntityState' is defined in an assembly that is not referenced. You must add a reference to assembly System.Data.Entity

I just got experience this issue with MVC5 in visual studio 2015 for web, finally I found solution adding entity framework to my project.
 

1.    Right-click on the References folder > Add Reference...

2.    Expand Assemblies on the left side of the window and select Framework.

3.    Scroll to and select System.Data.Entity from the list of assemblies.

4.    Make sure the box next to System.Data.Entity is checked, and then click 'OK'.

5.    Right click on the project and rebuild.