Extra lingot for that streak, wow!
Why not "you live only once"? Why should this be an error?
doesn't seem to be anything wrong with your sentence. although i will say that the answer given by duolingo is a more common way to phrase it.
I reported it.
I thought "se" was used as an "if"? "only if you live once"??
"Se" here is the reflexive pronoun. It alters "viver" so it's clear that the subject is the one whose life is the concern of the speaker.
How do you know that it means "You" ?
Só se vive - one lives only
In english it's possible to use "you" instead of "one" in cases like these.
or even "we" - "we only live once"