"kaczek" is the Genitive plural form. Putting aside that the exercise is about a singular duck, if it's just a phrase without a context, you should answer in Nominative - the basic, dictionary form.
So even if you had to translate "ducks", we wouldn't accept "kaczek" because it doesn't make much sense without a sentence, without a context.
The question is to spell "a duck" in Polish, using only the click-key letters presented. There is no "k" offered as an option to spell "kaczka"! The system will not allow me to skip the question. So I am in an endless loop, thereby ending my language studies on Duolingo. What an unfortunate programming error! Goodbye