Unless you were doing the reverse tree, you were out of luck to learn Korean on Duolingo.
However, rejoice, the Duolingo Korean course has been released yesterday. Only on iOS and Android for now, but the web release is planned for a near future.
Happy studying!
@Nigu84. I had been dying to do Korean and Japanese on Duolingo on my computer until they finally released it. I have started using Korean and Japanese on Duolingo, along with some other languages. 
I love using that site, but I noticed that some of the Korean alphabet signs are different from the ones on Memrise. I will alert Duolingo’s staff of that. Other than that it’s a great site. I did find that I prefer using Duolingo when practicing my Spanish and German, while with Memrise, I do my Korean and other languages that I am recently beginning to learn. It just seems easier for me, but I will continue doing all of them on both sites.
I was waiting for over a year on the Korean course… I was severely disappointed. I then found out about Memrise and haven’t looked back. However, I did enjoy learning German on Duolingo.
Yeah, but it all works out, because I made friends on here. 
And I’ve learned lots more, using this site.
Ehh, I’m not really a fan of Duolingo as a main learning source. It doesn’t really teach you vocabulary and grammar, and other things necessary for knowing a language. I’m also not a fan of the system of teaching, I like it where I can read something, look at a video, (or people speaking the language) and have more methods of learning, rather than just learning words and phrases. Duolingo doesn’t teach you the logic behind the language. I much prefer actual courses.