"It is hard to clean the house."
I think the English translation should be "Housecleaning is hard." I would say 家を掃除するのは大変です = It is hard to clean the house.
I just wrote: 家を掃除するのは難しいです。(without the kanjis because I don't trust Duolingo that much yet) and it was accepted.
What is souji? Clean? And 'hint' says kirei which is clean but doesnt offer it as an option.