Why cant table sugar be the positive control in test using Benedict's solution?

Table sugar, or sucrose, doesn't have an aldehyde group. Benedict's solution is used to determine if a reducing sugar is present. If it is a reducing sugar, the mixture will turn green/orange/red. The Benedict's solution contains copper (II) ions, which are reduced to a brick red precipitate of copper (I) oxide when the solution is heated. The Aldehyde (-CHO) group in a reducing sugar is the source of electrons that reduces copper (II) to copper (I). Since sucrose doesn't have an aldehyde group, it will not test positive for reducing sugars; it will not reduce the copper II in Benedict's to copper I and change the color of the solution.

Fructose does not have an aldehyde group, yet it is reducing, because it gets rearranged to the reducing glucose in basic solution.

All monosaccharides and some disaccharides are reducing sugars. Sucrose is one of the disaccharides that is not a reducing sugar.