I'm on a desktop... I just found the tips and notes 10 minutes ago. I had no idea where they were. Actually I didn't know they existed. (I've used the app quite heavily for over three days now)
So that's what it's like on a smartphone as well... Hours and hours of wondering "how am I supposed to know that?" and figuring it out on your own. Stimulating!
I'm not sure that's the best way to express it in English. You're saying the same thing of course, and there isn't even a Russian equivalent of "have" or "have got" in the original, but I think the "have got" construction is more idiomatic than should be expected in the answer. Every possible way of expressing something can't be accepted, try using the simplest answer you can. We are being graded by machines of course.
A possible alternative У меня не есть яблокo would seem to work, but is not accepted. I'm curious as to why.
The thing that bugs me about the "correct" answer (which seems very standard and common), is that it leaves open the possibility that, if I have "not apple", then I must have something else, like a peach. "Having" a not-thing is like having a unicorn.
"У меня не есть яблокo" sounds unnatural for Russian. "Не есть" is used rarely in such sense and for emphasis ussualy. Such sentence have to include subject. "Нет, у меня нет яблока." has no subject.
Examples of usage: