What is 'Player' in this context? The class itself? An instance?
Your code should work as long as neither 'healthLabel' , 'Player' and 'Player.health' is null.
If Player is actually a class and not an reference to an instance, you should have something in your script like
And then assign value to it via inspector or code.
After that, you can easily reference it in your Update method (example full script):
public UILabel healthLabel;
public Player player;
void Update()
{
healthLabel.text = player.health.ToString();
}
Cheers