Shouldn't "chef" (also) be in the translations? In my opinion, chef is an even better translation than cook. I feel a chef is someone who is paid to cook, while a cook is just someone who cooks at home. And since we are talking about jobs and professions here, I definitely think chef should be accepted as an answer.
"The cook walks over there" is accepted but you wrote (based on the report that appeared) "The cooks walks over there" which is wrong obviously.
Please double (or triple) check your solutions before unnecessarily reporting wrong solutions. Also, my impression is that we could spare a lot of time and confusion if you read this: https://forum.duolingo.com/comment/38935053 and perhaps look around at the forum in general.
I am definitely a naive duolingo user. You answered my comment on the 5th and it wasn't till today, the 22nd, that I found out I get answers on my e-mail account (I don't check it that often, obviously). I apologize right now for all the comments I shouldn't have left if I had been able to read your hyperlink earlier.