For what it's worth, and given that I don't know which Skill you're working on, I'd say "probably," because DL USUALLY uses "a" in the absence of any kind of demonstrative. So it MAY just have been overlooked, if the (hidden from us but known to The Powers That Be) context of the sentence happens to be a discussion with someone about where the post office is. You could also Report it, if you haven't.
What I meant to say was that IMO the English translation could be OK with "A post office" (in addition to with "THE" post office), since there's no demo in the original.
But the sentence "A/The post office is left of the museum" also would be fine, along with "A/The post office is TO THE left of the museum."
Interestingly, ngrams shows "left of" as more common than "to the left of" (if I did this right...) https://books.google.com/ngrams/graph?content=is+left+of%2Cis+to+the+left+of&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cis%20left%20of%3B%2Cc0%3B.t1%3B%2Cis%20to%20the%20left%20of%3B%2Cc0