Taiwanese and probably a lot of southern Mainland Chinese tend to say 哪里, while northerners typically say 哪儿。那里 and 那儿,这里 and 这儿 are the same difference.
What the actual heck i gave 你在哪里 yet they said I got it wrong. I even matched with the "correct answer" which they provided under. Which completely matched the ones I chose. I lost my only heart and now i have to restart. Is this a joke Duolingo?