In english we don't have a separate word for a plural you, although the southerns have something close with y'all, as in 'how y'all doin'? In the northwest, we commonly say 'hey, you guys, what's up?' But spanish does have a word for the plural 'you all' or 'you guys' in the form of ustedes, not to be confused with usted, which is just the formal version of the singular you. In the sentence above, the person is most likely addressing a family or maybe talking to someone in town, asking do you all, the people of this family or town, have a pool. Duo used to say you all, so there was no confusion, and also demanded you answer with the same, but doesnt any more, but if you answer 'you all', it will be accepted, but 'you' is all that is required to get it right.
I've never had it reject swimming pool, if it did, report it. However, look closely at your sentence, there may be something else wrong, and when it corrected the something else, it used swimming pool. Duo lets you say pool, because in english, we often just say pool when talking about a swimming pool. If we are talking about something else, we will specify, like 'a pool of milk on the ground.'
Regarding the plural form... the speaker may be addressing to a group of persons, so the subject will be the plural second person ("ustedes" or "vosotros") and not the singular second person ("tú" or "usted").
Notice that some countries predominantly use the "ustedes" form while others use the "vosotros" form and there isn't actually a distinction between formal and informal in the plural form.
Regarding the singular second formal and informal person, it is country dependent: on some Spanish speaking countries there is no formal/informal speech, that is, on some countries everyone uses "usted" even in informal situations.