I won't take "beef steak" as an acceptable answer, yet English-Italian dictionaries quote it as the correct translation for bistecca.
You get marked wrong here for being more accurate with the answer. That's Duolingo, and it's a common thing on many courses. Simply not enough fair and reasonable tolerance in the English answers in any course.
Might it also be: I am eating the steak. When is it necessary to use an article with an item to indicate the entire universe of such things, as opposed to the meat before me now? i.e., why is it not I am eating the steak, as opposed to I (engage in such activity) eat steak....?