This is tricky because there is no literal English translation. I thought it for many minutes but still got it wrong. I think the best translation would be "It is not possible to pass by car".
If you mean "you" as in specifically talking to one person, then it would be puoi. If you mean "you" as in the general population, or "one" in formal English, that is when you use the si puo'.
"in macchina" can be "by car" but can also be "in a car". Consider the following. (1) From an article by Paolo Foschini in Corriere della Sera (14/04/2007): "Finché gli agenti decidono di caricare madre e figlia in macchina e le portano via". The mother and daughter are not loaded "by car", they are loaded "in(to) a car". (2) from an article by Claudia Voltattorni in the same newspaper (03/10/2007): "come capire che chi entra in macchina e ha un alito sospetto è anche la stessa persona che guiderà l'auto e non uno dei passeggeri?" Clearly, "entra in macchina" means gets "in the car". (3) From L'Espresso magazine (3/20/2004): "Oggi posso mettermi in macchina e sapere precisamente quello che sta accadendo a elementi con i quali non ho nessun legame fisico" Clearly, once again "in macchina" cannot mean "by car". I have a multitude of examples, but that is perhaps enough of a lesson for duolingo's computer for one day.
So the "Correct Translation" suggests the sentence can also be You cannot go by car; is that correct?? If it were "you" then you would use "puoi" right?
"Go by car" is the kind of loose translation that Duo sometimes does but doesn't let you do. Non si puo' is an impersonal construction, something like One cannot go, or You (in an impersonal sense) cannot go.
I put 'one cannot go by car' and it was accepted. I guess I was lucky this time!
Okay, I'm missing something here. I keep making the same mistake, automatically translating "podere" as "being able" instead of "can", but I see them meaning the same thing here. Is there another construction that means "be able" in Italian?
"Potere" means "can" or "be able". There's no difference. Other Italian expressions are "essere capace di" or "riuscire."