simple syntax error, in the bottom 3 you are including the string in the rect constructor. place a ) after the 20’s and remove one from the right end of each.
Rect has a Constructor that takes 4 parameters. ( see Rect Constructor for more information)
in your code you give too many parameters to the Rect Constructor.
Gui.Box(new Rect(45,30, barLength,20,currentHealth.ToString("0")+"/"+maxHealth));
the fix is simple. all you need to do is remove one parameter. i guess this is what you wanted to do:
Gui.Box(new Rect(45,30, barLength,20),currentHealth.ToString("0")+"/"+maxHealth);
notice that the Rect constructor now takes 4 parameters