What is the mass of a person weighing 600 N?

Weight = (person's mass) x (acceleration of gravity in the place where he is)

You can see that it all depends on where he was weighed, because

the same mass has different weights in different places.

If he was weighed on Earth, where gravity = 9.8 m/s², then

600 N = (mass) x (9.8 m/s²)

Mass = 600 N / 9.8m/s² = 61.22 ... kg (rounded)