A comparison with English might be helpful.
The letter “I” can be read in various ways depending on context, including あい and い.
The word “I” can be only be read あい.
If WK would ask you for the reading of the letter “I” neither あい nor い is incorrect, so it’ll just shake the screen at you until you answer the one it expects.
On the other hand If WK would ask you the reading of the word “I”, answering い is incorrect and it should and will fail you for it.
In the same way, while にち and ひ are both valid readings for 日 as a kanji. You cannot pronounce it にち as a word.
EDIT: i.e. you can’t say today is a good day by saying “今日はいいにちです”