I came across the same issue today whilst working through an SAE II exercise.
Shortly after it happened, the site advertised that it was down for maintenance. I suspect that some maintenance activities began before the message was displayed, hence why it couldn't find key references...
The error you get is because you clicked on the class name (with the green leaf) directly. This opens an instance of the said class. But since BackgroundCheck does not have instances, it is not setup with a key.
Click the triangle expand icon instead to locate the rules within the BackgroundCheck class, instead of directly clicking on the class itself.