I see your frustration; I have to remind myself of this when I get frustrated by the app: fixing this specific problem likely takes more than a few seconds. The hints are probably not done one exercise at a time but by a set of algorithms that would need to be restructured-- which would affect the entire Chinese section of the app. Though hover over accuracy is important, I imagine that Duolingo software engineers prioritize putting out fires to keep the app running. After all, Duolingo is comprised of humans that have to prioritize problems like the rest of us.
(And lets not forget, "it would take seconds" for users to look up a phrase elsewhere when Duo provides exposure but lacks in accurate reasoning.)
If you want to say "two people" when entering the restaurant, you can say 两个人=两位, which are both common for me.
个 is a general classifier, while 位 is specifically for counting people, and is more formal. Actually 两位人 is technically correct but nobody says like this, perhaps because 人 is redundant given that the classifier is 位.
I was taught that using 个 as a measure word for people was very rude because its used to measure such common things. For people, the correct measure word should be 位 because its more respectful. However, as others have pointed out, no one says 两位 outside of restaurants and you can say 两个人 without any problems
You know 位 is a measure word here from the use of 两 instead of 二, and as a measure word, 位 can only refer to people in a restaurant context (number of guests, honorific). Otherwise it could also be a measure word for binary units (bits), but I think its clear that we are not talking about 2 bits here (and anway, even in a tech context, who would talk about 2 bits anymore... the era of 8-bit file names is over :D ).
"Two persons" is equivalent to "two people" in English and should be accepted. I understand that 位 is a counter for human beings, so an actual word for a human being is not necessary. But "two places" should also work.. More colloquially, in English one might say "table for two" or "we are two" and that would also be acceptable both grammatically and socially. This is a beta version of Chinese, and I am grateful for the thought and effort that has gone into its development. It is in many ways very clever, but Chinese is extremely simple grammatically, allowing for a lot of ambiguity--even regarding singular and plural, so DuoLingo still needs to add more translation options, because being marked wrong for correct translations is very frustrating--even if I know that this is just an automatic computer response.
I disagree that two persons is equivalent to two people. Persons is normally only used to refer to legal persons, not to count human beings. I can't imagine a native speaker asking for a table for two persons. It might technically be a possible rendering of this phrase but it's not idiomatic English.