"It takes one day."
A theory says when time indicator is used like an adverb, it does not require a particle.
It takes a lot of time.
It takes 3 hours' time.
I guess Japanese uses it so naturally that there is very little discussion about it. it seems to be only confusing for foreigners. Anyway learn the pattern by memorizing it first and don't be too astonished when you see cases with a particle after time, which can be は, で, が, に and others.
In every case the kanji is read the way duo wants it typed. That's what makes people frustrated. Then in some places, they ONLY accept the kanji.
The worst is days of the week. 月曜日 is wrong and so is げつようび. Instead, it needs to be typed it in three parts, makings sure that the middle part is in hiragana instead of kanji while the first and third parts are in kanji, not hiragana.
The Japanese learning English course is much more lenient.
EDIT: They've started accepting 曜日, so that's nice.
TL;DR : In this case, they probably used hiragana because they wanted to indicate the correct pronunciation, given they don't support furigana.
From previous comments:
... one day's time is いちにち, but 1st of the month is ついたち
However, both would be written 「一日」 in kanji. If you have a Japanese input on your keyboard you can try it. Duo has historically had a bit of trouble with Japanese because it doesn't support furigana (a reading aid where kanji are accompanied by the correct pronunciation).
Also, the characters used there are hiragana, not katakana. Katakana is typically used for the transcription of foreign words into Japanese (though it has other usages). Whereas hiragana, from Wikipedia:
Hiragana is used to write okurigana (kana suffixes following a kanji root, for example to inflect verbs and adjectives), various grammatical and function words including particles, as well as miscellaneous other native words for which there are no kanji or whose kanji form is obscure or too formal for the writing purpose.
If you came across this sentence from a Japanese source, it probably would've just been 「一日」or even「１日」and the correct pronunciation would've just been implied.