"What time will you open tomorrow?"
I would like to say by native instinct we don't omit 你們 here, e.g. if we are talking to a shopkeeper and are obviously asking when the shop opens again tomorrow. We omit it only when we are actually on the topic of opening schedule already, e.g. the shopkeeper has already suggested that we come again tomorrow.
It is correct, but I assume duolingo doesn't teach it, so it doesn't accept it since you're not supposed to know it (considering you just use duolingo). I have learnt that 什么时候 and 几点 have amost the same meaning, but 几点 is more specific. Again, what you said is correct but duolingo simply doesn't recognise it.