I answered "das Heft" whilst translating "the magazine", and was marked incorrect. Should this be an acceptable response?
In my opinion, "das Heft" should not be accepted for "magazine" because it is understandable only in contexts in which it is absolutely clear what you are talking about.
I think that the prevalent meaning of "das Heft", without any context, is "notebook", especially for students, "exercise book". If I tell someone that I want to buy "ein Heft", they will most certainly understand that I need something to write in.
In my opinion as a learner, Duolingo should teach us vocabulary that makes it easy for us to communicate; this includes a sufficient degree of precision of the words used.
To exaggerate a little: In almost any colloquial contexts, any object can be called "das Ding": "Gib mir mal das Ding rüber" will be totally understandable if you are in the situation.
But it would certainly not be a good idea to accept "das Ding" as a translation of, for example, "screwdriver" just because you could use that word colloquially in particular contexts. ;-)
Edit: It just occurred to me that "das Heft = magazine" is actually wrong in some contexts. Example:
In my spare time, I love reading magazines.
should not be translated to:
In meiner Freizeit lese ich gerne Hefte.