"What time will you open tomorrow?"
I feel like "明天几点开门” is an acceptable way to convene the same meaning. Perhaps even more representative of everyday conversation.
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.
So, you wouldn't say 明天你几点开门 even if you were talking to the shopkeeper who doesn't have any other employees?
This is acceptable since the context is clear to the translators. However, in Duolingo, the word order is important.
I have suggested the singular version of “you”. I imagine the plural version is more natural in a large shop, but a “sole trader” would be a singular “you”.