I can't find my "Show Mask" on Lumetri color

Technically it's a selection, not a mask, since you don't draw it with any tools. You make the initial selection by using the eyedroppers to pick the flesh tone you're trying to modify. When you have the color/grey box selected it typically shows an all-grey screen until you make a selection; I think what's happening is that you haven't selected the forehead yet (all the sliders are on a bluish tone) and you also have the INVERT selection button on (the little blue box) so it's not showing you anything. I would select his forehead with a dropper and turn off the invert switch; you should then see a grey area around the color you selected.