"Where are you?"
Their meaning is simply different. 那 is "that", 哪 is "which". Combining with 里 gives you 那里 "there / that place" and 哪里 "where / which place".
My simplistic explanation is that 那里 means "that place" and 哪里 means "which place".
Remember that 口 in front of a character like "吗" or "吧" or "呢" are questions words (not a rule for all words) but it helps to remember that 哪 is asking something because "kou" means mouth so I remember that way.
I ask to myself for 1 minute, where ❤❤❤❤ was the mistake ...damn chinese is like a rebus
My answer was exactly the same as the correct answer, but it was marked as incorrect.