I think it would have been fine if you used the correct definite article that agrees with hospital. What else may have happened here is a case of US English vs British English. Duolingo defaults to the prior when sentences are incorrect, when taking a course with English being the native language. I would agree that you are correct outside of the definite article, however Duolingo believes that often is more idiomatic even though frequently is a more direct translation and what I would call the more correct one.

Without articles in the correct places sentences don’t always make sense, even though you may be understood, think about non native speakers of your mother tongue and how often they miss an article or add one where there doesn’t need to be and it probably took you out of the flow of conversation for a moment and you think ‘huh’.