I think the issue here is that you do not need plurals for general affirnations if you are not talking about a specific group (i think i read that somewhere). So since they are not specifically talking about someone, the singulsr version of the word is accepted. Think about other duolingo exercises where generic phrases about men and women were also writteb with 남자 or 여자 without the 들
I feel like a broken record. DL needs to provide some context if they are going to not accept singular or plural, "a" or "the", 이/가 or 은/ 는. It is obviously a source of great frustration for all who take the course, and leads to questioning the credentials of the developers when obvious correct answers are not acccepted. It is something that could be easily remedied by adding "context clues."