Filtration is a method that is used to separate insoluble solids from a liquid. Since salt and sugar both dissolve in water, filtration cannot be used to separate the two. When sugar is mixed with water it creates a homogeneous solution, meaning you can't see the individual particles, unlike when you mix sand with water. The sugar water is a solution because no chemical reaction occurs, but to separate it you need to create a chemical reaction by distilling the liquid.