The project is likely corrupted. Make sure that the version of Unity you have is > the version used to build the Google Cardboard SDK.
If that's not the problem, close Unity, delete the current Google Cardboard SDK project and plugin. Download fresh version here.
After downloading it, import the GoogleVRForUnity.unitypackage file. This should fix your problem.
EDIT:
Things changed in the latest Google VR SDK update. The VR Demo Scenes now require VR
Technical preview to work. You can get the latest Unity version for VR here. Scroll down to the end of the page.
The latest version 5.4.2f2-GVR10.
Once you download this version of Unity UNITY_HAS_GOOGLEVR
will be defined in Unity and you can now run the ControllerDemo demo.
Note:
You can run the DemoScene scene in Assets\GoogleVR\DemoScenes\HeadsetDemo directory without downloading the Unity GVR version. Right click on the Project tab then click Reimport All.
You just cannot run the ControllerDemo scene in Assets\GoogleVR\DemoScenes directory without downloading the Unity GVR version.