You can only cast to another class which is the parent of your object’s class. Obviously. You can cast an Animal to a Dog if you know it’s a Dog, but you can’t cast a Duck to a Streetlight - they are irrelevant to each other. Here you are casting PlayerController to some UI, which is not a PlayerController for sure. You have to cast it to your own PlayerController class, and get the Score out of that.
PS: Your Score is not an int, it’s an object, as it’s blue. Integer would be green.