"The restaurant has twenty one cooks."
Translation:Restaurangen har tjugoen kockar.
I am not sure, but it is wrong maybe because we talk about an en word. Correct me if I said something stupid!
I read about this in another comment. Unfortunately I don't remember which one but I remember that it was not because of en words.
I think it was something like
tjugoen is used when the noun is countable
tjugoett is used when it is about age
Edit: Found the link: https://www.duolingo.com/comment/6378354 Thank you HelenCarlsson for clarification. :-)