Can you put sheetrock over tiles?

You cannot use sheetrock as flooring, period. If you meant on a wall, there is no reason you cannot put sheetrock over tiles. The question is why would you want to! Why not remove tiles and material behind, which is probably sheetrock, and then install new sheetrock. If it is an area prone to moisture you could use moisture resistant sheetrock. The job would look better and you would not lose the space by adding over existing material.