In the Space Shooter tutorial, gameController.GameOver has a problem, something with it doesn't exist in the current context?

I was following all the steps in the Space Shooter tutorial, when I am on the Explosions video, and the code is not working. I copy and pasted just as the video had, and now my game won't enter play mode because of gameController.GameOver. Does anyone know how to fix this? It'd be really appreciated, I need this for a project. Thank you.