Any reason "population of Kiev" is not accepted?
I mean "of" isn`t accepted, too
Not accepting 'Kiev' seems to be a conscious decision. At least it seems it's not accepted in lots of sentences in this course.
Well, and for another reason it forses me to spell Odessa with double s.
Despite the fact that both toponyms have these exact Ukrainian forms due to the specifity of the Ukrainian language
Київ is the City of Kij (Кий). Київ/Киев-град.
Одеса is spelled with one с due to the rule of smooth sound (милозвучність): for the same reason, many other words lack double consonant, e.g.: Росія, піца, професор, інтелігенція and so forth.
So I don't see a reason to force the use of any specific version in English. Let it accept both.
They'd better not accept nominative case instead of vocative: Мама istead of Мамо, when being obviously referred to.