"He is a doctor."
Possibly because you left off the accent -- I'm not sure how strict Duolingo is with that.
Otherwise -- what was the entire sentence that you wrote?
Just γιατρος obviously is not a translation of "He is a doctor".
I wrote "Αυτός είναι ένας γιατρός" and was marked wrong with correct translation given as "Αυτός είναι ένας ιατρός". Given that γιατρός is an alternate spelling of ιατρός shouldn't the first be accepted?
I wrote: είναι ένας γιατρός. Why is this wrong? I learned only use Αυτός when you want to emphasize 'he'
Sounds way too indefinite in Greek to say αυτός είναι ένας γιατρός, (because the person is already introduced, you just said he is...), to the point that saying this phrase with the right expression would result in irony (degrading the fact that he is a doctor, actually that he is one of the many doctor in this world), because in Greek ένας/μια/ένα is not only the indefinite article, but also means "one", and they are not really separate, so there is no reason to specify that he is "one" doctor, he couldn't be two after all. So, it's not a really native way to say "Αυτός είναι ένας γιατρός", it is used in the cources in the first lessons for education purposes. Ένας γιατρός is used mostly if you want to talk about a doctor that the person you speak to does not know, or that you do not wish to specify, for example: There is a doctor in athens that...=Είναι ένας γιατρός στη Αθήνα που...
Marking me wrong for 'Αυτός είναι γιατρός' & suggesting only 'ιατρός' would be correct. This doesn't make sense. I have seen 'ιατρός' but I'm sure that 'γιατρός' is actually much more usual.