"we need bread" why is it: 'il faut du pain' and not 'nous faut du pain'
“Il faut” is an expression that might be translated in idiomatic ways into English. “Faut” is only used in the third person.
Here is an article with some examples: https://www.thoughtco.com/french-verb-falloir-1368847
falloir = "to be necessary"
il faut: "it is necessary"
il faut du pain: (lit.): "it is necessary (to have) some bread" -> "some bread is needed"
il nous faut du pain: "it is necessary for us (to have) some bread" -> "some bread is needed for/by us"
important to note that "nous" here is not acting as a subject pronoun (we), but rather as an indirect object pronoun (to/for us). Nous appears the same both as a subject and as an object, but compare this same expression when replacing nous with je/tu/il:
il me faut du pain -> I need bread (lit: "it is necessary for me (to have) bread")
il te faut -> you need bread (lit: "it is necessary for you (to have) bread")
il lui faut -> he/she needs bread (lit: "it is necessary for him/her (to have) bread")
Another thing worth noting is that falloir is defective, meaning it doesn't conjugate into all forms. The verb only exists in the 3rd person singular (il), and so will only ever be expressed as such, unlike (nearly) all other verbs, which change form to reflect speaker:
nous mangeons (etc.)
but only ever
Falloir and pleuvoir are the two common "defective verbs" in French - by "defective" I mean that they exist only in the 3rd person singular. If you think about it "to rain" (pleuvoir) is defective in the same way in English as well!
relox84 has explained above how to relate "falloir" to "us" but I cannot imagine the circumstances where you would ever need to do the same for "pleuvoir".
Did you check out the forum message for the sentence? There's usually a long discussion about just this issue.
Link to it from the page that tells you if your were right or wrong.