Sugar-free yogurt depends on whether it is "sucrose-free" or really "sugar-free". Many yogurts on the market are "sucrose-free", which means there is no sucrose but xylitol or acesulfame K, so it is still sweet to eat. Some yoghurts are really sugar-free and not sweet, but they taste worse and are very sour. I have made them myself without sugar, and they are very sour. There are also yogurt that is not sweet on the market, and I have bought it, which is also very sour.