Vacancy of electron created in the valance band of solid due to the upward transition of electron is called hole. In case of semiconductor valance and conduction bands are separated by a gap and hence electron can make transition and hole created.
But in case of metal the valance and conduction band overlap with each other, hence valance electron need not have to leave valance band to participate in conduction. It can do so only by changing its energy level and hence holes are not generated in metals.
Metal flat bar