Using this lever puzzle, how would I make my piston door turn OFF when the puzzle is correct? [MC1.14.2]

All you have to do is remove the signal inverter that uses a redstone torch. Here it is in the video tutorial (at 5:30):

Redstone inverter.

All you have to do is replace the torch with redstone, like this:

Like this.

As you can see, the doors are now open, even though the visible redstone is powered off. You can do everything else as it is in the video.