Translation:The role of teachers is crucial.
So I interpreted this as "The professors' paper is fundamental." as in the particular professors have written a fundamental research paper on a topic. In my work as a research engineer, that is a common sentence to utter.
I dunno if you are portuguse, but it is not common. in Brazil at least. We don't call researches "papers" here. Role is the perfect translation in this sentence.
I'm American. We call an article in a research journal a paper. I didn't say that sentence would be common in every day speech, but it would be common in the research community. My question is then, how do you say" the professors' paper is fundamental" in Portuguese?
So I said the role of THE teachers is crucial and they didn't accept? Surely the dos here implies specific teachers? The role of teachers (in general) would be o papel de professores?
I believe DuoLingo ignores punctuation (try some experiments). It would take a lot of not-very-productive coding to change that. Meanwhile we can provide answers that don't depend on punctuation for correctness.
The word "papel" can mean both "paper" (the substance) and "role", so technically "teachers' paper" is not wrong but "é fundamental" at the end of the sentence suggests "role" is the best choice here.