Use allulose. It is the only common sugar substitute that lowers the freeze point the way sugar does, which is what slush machines actually need. Stevia, erythritol, and monk fruit blends taste sweet but freeze like water, which is why diet soda comes out as ice or stays liquid. About half a tablespoon of allulose per 8 oz of liquid does it.
The machine needs dissolved solids to make soft slush; that is the job sugar does. Artificial sweeteners provide sweetness with almost no dissolved solids, so a zero-sugar mix sits below the 4% threshold and the machine refuses it (downward flashing lights, beeping).
Allulose is a rare sugar with roughly 70% of sugar's sweetness and almost no calories, and it depresses the freeze point like the real thing. Dose at about 1/2 tablespoon per 8 oz of liquid. Powdered allulose dissolves more evenly than syrup in cold mixes.
One 24 oz bottle of zero-sugar lemonade plus 3 tablespoons of allulose is a community-tested starting point. Taste before running; add sugar-free syrup if it needs more sweetness.
No. Stevia and erythritol do not lower the freeze point, so the machine cannot make slush from them alone. Allulose is the substitute that works.
About half a tablespoon per 8 oz of liquid. For a 64 oz batch, roughly 4 tablespoons.
Not by itself. Add allulose to bring the dissolved solids up, or blend it with a real-sugar mixer.