Does liquid and gases have centre of mass.If no why?

Yes liquids and gases have center of mass.

You may find the center of mass of a container of liquid or gas the same way you find the center of mass of a solid. The difference is that as the "shape" of the liquid or gas changes, so does the center of mass. A liquid or gas that is held in the same shape throughout an activity will have the same center of mass as a solid of the same shape.