You probably got logged out of Duolingo halfway through the lesson, that happens to me sometimes.
Does this senctence mean, that apples fall off a tree, or does it have another meaning?
Without context and further specification it kind of sounds like really everywhere. Like in this animation movie "cloudy with a chance of meatballs"
More naturally probably is "...everywhere around the tree" though.
But you probably anyway meant something in the style of "pigs can fly, when the apples fall everywhere"? No idea. I don't think so.