I put 'Please cook separately' because I thought that sentence ending had a more polite feeling, am I wrong?
Yeah, I found it odd that they used 세요 AND put "please" in the word choices and still don't accept it. But that was my answer too.
-(으)세요 is the polite imperative mood but it doesn't really imply the word please. It's just the polite way to say it to someone.
Then how do you ask someone politely to do something in English? Generally, you add the word "please". : )
Though, I will say, both in Korean and English, the politeness of your sentence increases with length.
"Cook separately." --> "Would you mind cooking that separately, please?"
따로 요리 해. --> 따로 요리 해주시기 바람니다.
guys don't learn with doulingo you won't learn nothing with this app if you want to know the korean language than learn the grammar first