"We do not have it."
Translation:O bizde yok.
This sentence appears not only in the Locative lesson (for example, I encountered it while strengthening all skills wit "Strengthen skills" button). Therefore, you do not always have the context of the Locative lesson. So, if "bizim onumuz yok" is grammatical at all it should be accepted, too.
I think that you can't use a genitive-possessive-var/yok sentence here because in this context, "it" is always going to be something that has already been mentioned, so you need a "have-the" sentence instead of a "have-a" sentence. Put another way, this is about the location of the "it" -- not the ownership of the "it."
[I agree that you'd have to use the the correct possessive ending -umuz if you wanted to say this. But probably to say "our it" you would just say "ours" with bizimki.]