I think this is too restrictive. As a native speaker, I rarely, if ever say "What do you do?", it's one choice among many. Other people say it, but it's not a universal. It's also possible to say "What job do you do?", "What do you do as a job?", "What is your job?", "What do you do for a living?", "What do you work as?", "What work do you do", "Where do you work?"
Duo puts too much emphasis on trying to translate the Turkish into English. When you learn a new language, you shouldn't be translating, you should be learning to think like Turkish people think. There is nothing wrong with reading this Turkish as "What job are you doing?" because that is thinking in Turkish instead of English.
I am British. I've heard 'what do you do' in American films but never used it or heard it IRL. In Britain we would say it the same way as the Turkish (a direct translation). Why have this as 'incorrect' and a non-literal American colloquial way of expressing the same question as 'correct'?
Ne yapıyorsun is what are you doing? Adding iş would make it what job(work etc.) are you doing?
Oftentimes Duolingo requires us to give a translation which accurately fits the Turkish even if it is not a sentence that would normally be used by a native English speaker so this should allow what job are you doing.
I hate how restrictive this course is. I'm a native speaker of English and fully understood the colloquial meaning of the sentence, but put "What work do you do" because I thought it would count "what do you do" as wrong, as it's too colloquial in English. What I put could also be said. There are so many times where I feel like my answers depend not on my understanding of the language but on guessing correctly which exact way the course wants me to respond :(