since antimatter doesnt occur in nature(or present in minute amounts in space), we have to manufacture the particles artificially, and that involves ALOT of energy, possibly way more than the annihilation of matter and antimatter can produce. for that reason, antimatter is only produced in laboratories for studies.
furthermore, storage is a huge problem; antimatter is held in check by strong magnetic/electric fields that prevent it from flying off, and that requires alot of energy too.