Sorry, the decision of the administrators of this course was political. They decided to accept only Kyiv.
My own opinion is different, I would accept "Kiev" because it's a historical name that many foreign languages use. In Ukrainian there are plenty of Ukrainian names for Polish cities, we use them instead of the proper Polish ones with accurate pronunciation. And nobody is making a fuss about that... Also, it's easier for foreigners to pronounce.
However, I totally understand the other side. Some people are very sensitive to these kind of things. They think the spelling of one word will influence how the country is perceived.
And the latter ones are right. The city is called Kyiv, it doesn't matter who feels it hard to pronounce. Moreover, it is hard to remain apolitical when your country is in war, engaged by the evil neighbour, thus I don't want world to use the name, provided by that neighbour. The city of Kyiv is founded by Kyi, and the name Kiev arose just because russians can't say Ky, as they don't use such syllable.