The Ninja Slushi uses its temperature-control LEDs to tell you which direction the mix is wrong. Lights flashing in a downward sequence with a beep every minute means not enough sugar (under the 4% minimum). Lights flashing in an upward sequence means too much alcohol or sugar. Both are recipe errors the machine wants you to fix, not faults.
The mix is under the 4% sugar floor. Stir in 1 to 2 tablespoons of sugar, honey, maple syrup, or allulose per serving, and let it keep running. This is the classic result of using zero-sugar soda or unsweetened juice.
The mix is over the limit on alcohol (16% ABV cap) or extremely heavy on sugar. Add water or juice to bring it back into range. Recipes near the cap, say 14% ABV, can trip this warning as they chill, so leave headroom.
If it starts complaining within the first few minutes, it read the mix as unfreezable from the jump. Recheck the recipe against the sugar floor and alcohol ceiling before assuming a defect.
Under 4% sugar in the mix. Add 1 to 2 tablespoons of sweetener per serving. Allulose, sugar, honey, and maple syrup all work; stevia and erythritol do not.
Too much alcohol or sugar to freeze. Dilute the mix with water or juice. Keep the final mix at or under 16% ABV.