In a web-based strengthening exercise, I was asked to write "April" in Greek and was red-screened for responding with "ο Απρίλιος." If elsewhere in the exercise, the phrase "October is a month" was given as "Ο Οκτώβριος είναι ένας μήνας," I think my answer with the definite article ought to be accepted, too.
There is a difference between individual words and words in sentences. Yes, I've had exercises rejected for the same reason. We're so used to using the article with the noun tha we do it automatically but the computer program that grades the answers only knows the one word it was programmed to accept.