The nutritional yeast really enhances the cheesy flavor in this sauce. While you can omit it without completely ruining the recipe, I highly recommend keeping it for the best cheesy experience. If you can't find nutritional yeast, you can use brewer's yeast as an alternative. If that's unavailable as well, you can substitute it with a mixture of ¼ cup (60 milliliters) of beer and ¼ cup (60 milliliters) of unsweetened non-dairy milk instead of the ½ cup (120 milliliters) of milk specified in the recipe. Another option to consider is adding a bag of vegan cheese as an alternative.