What does "I wanna beat you when you can't blame it on" mean? Why she used negative sentence after "when"?
In this case, the full sentence is "I wanna beat you when you can't blame it on the broken nose or the buzzer or the fact that you thought you might be getting mono." This sentence means that she wants to beat him in the game fairly and have him accept defeat. In the past, she has beaten him but Ross has always made excuses for losing, so she wants to win without having him make excuses. She uses "can't" after "when" to describe a situation where he "can't" have any excuse for losing.