No, just "sky" is an accepted answer to this question. However, while we can talk about e g "seeing some sky", "a foreign sky" or "purple skies" (or use just "sky" in compounds and collocations) I suspect that "the sky", as "the thing above our heads", is by far the most common translation, and in that case it does need "the".
You're right that the kanji 空 can mean sky or empty, but when it means "empty" it's not read as そら (sora). I'm not sure if you had a different question, but I'm seeing this question in hiragana, so "empty" wouldn't be an accepted answer for そら.
If you had a "translate this into Japanese" question, I'm surprised 空 wasn't accepted. If you submit an error report, it will be.
If you had a "type what you hear" question, there's currently a conflict between the duolingo programming and the nature of the Japanese language that results in only one "correct" answer being accepted, so only そら would be marked correct until they can figure out a solution.
Thank you for the info. I no longer remember which type of question it was. I tried to submit an error report, but the options didn't include "my answer should have been accepted"; there were 3 unfamiliar options that I don't remember anymore. (Oops. Should've taken a screenshot!) I guess maybe that means it was a "type what you hear" question; this is the first time I've wanted to submit an error report on those.
It probably won't be the last... Those are one of the few question types where you can't submit a "my answer should be accepted" report, so a lot of users are just using the word bank for the "type what you hear" questions for now.