"In the morning I finally read that short story."
Translation:Утром я наконец прочитал этот рассказ.
I used the imperfective "читала" and was marked wrong. The English could be translated either way, I believe, since it's not clear whether the story was completed, which the perfective suggests. If you are trying to force the use of the perfective, why not change the English to something more like "In the morning I finally read the rest of (or all of, or finished) the story"?
I would say it sounds awkward because of the "finally" + past tense which implies that the action has been completed (at least for now). Sorry I don't know how to best explain this, but because there's a "finally" there, I don't think читала is the appropriate verb to use.