ん is a weird sound. It's a uvular N, pronounced almost like an ng in English but without fully closing the back of your mouth. This means that んよ sounds very different from にょ.
(Also, fun fact: tempura（ 天ぷら） is pronounced similarly in Japanese as in English because ん followed by a p sound turns the ん into an m sound! It's much easier to pronounce. Try saying "inpressive" or "anperage"!)
Unless I am missing something it is completely random when kanji, kana or kanji+kana is required for a correct answer. In this particular case 金曜日 was NOT accepted?! Having to commit to memory which particular answer is correct for a given question is really annoying and a waste of brain power. One solution could be to accept both kanji and kana answers.
The combinations of correct answers are fixed by the course contributors. In this case the contributors thought that the kanji 【曜】 was too difficult as per the standard JLPT N5 level, thus it was excluded from the initial correct answer. The contributors need to enter each combination of answers manually in the Duolingo database to make that particular answer correct, but it seems that although a number of feedbacks have been sent, nobody has taken any action and I have never heard of any reasons from them. It may be fixed in the upcoming 2.0 course update because they promised a lot more kanji are included.