It's because the clue/question is phrased slightly confusingly.
All of the other clues are phrased such that they refer (where applicable) to a country that beats Croatia, whereas that clue refers to a city. So it's easy to get thrown.
At least it was for me - I also had the same problem of trying to type in the city for the longest time and wondering why none of e.g.: Zurich, Zaragoza, or (uhhhh...) Zaporizhzhiya? were working, before finally realizing my error in the dying seconds of the quiz.
Personally I think it would be better/easier if that clue were phrased along the lines of "Country containing the most populous European city starting with Z."
All of the other clues are phrased such that they refer (where applicable) to a country that beats Croatia, whereas that clue refers to a city. So it's easy to get thrown.
At least it was for me - I also had the same problem of trying to type in the city for the longest time and wondering why none of e.g.: Zurich, Zaragoza, or (uhhhh...) Zaporizhzhiya? were working, before finally realizing my error in the dying seconds of the quiz.
Personally I think it would be better/easier if that clue were phrased along the lines of "Country containing the most populous European city starting with Z."