Translation:A minute is less than eighty seconds.
Probably because you used the word "long". Remember, according to Einstein's theory, time is relative and is not an object that can be measured with a ruler.
You could probably say "a minute has less than..." and it would be marked as correct. However, if you tried something like "a minute is comprised of less than ..." then I think it would be marked as wrong. Although they both mean the same thing just remember, you're being marked by a computer. :)
I'd be comfortable with "a minute has fewer than 80 seconds" or "a minute is less than 80 seconds." Incidentally, it seems to me that the non-pedantic often use "less" instead of "fewer" but seldom use "fewer" instead of "less," as though having fewer words would make a language simpler and more international.