Kulikuwa na kiti kimoja doesn't sound as awkward.
That's what I would write too. Report it.
What's wrong with Kilikuwa kiti kimoja? Marked as incorrect.
Kiti is the subject of kilikuwa, so it should come first. But people are also saying that the sentence isn't very natural anyway, so I wouldn't worry too much about it yet. Hopefully they make the sentence a bit more natural first.
Should it not be kulikuwa to say "there was" kilikuwa kiti makes it "it was"??