Why we are not able to store AC ??? but we can store DC.

Really? Flywheels driven by AC motors store rotational energy which in turn can be used to drive an AC generator. Pumped storage uses AC Motors to raise water into a reservoir at night so that it can be released to drive an AC hydrogenerator during the day.

Sadananda,a good question from a curious mind! In addition to the explanation above,DCs are produced mainly from chemical activities as in the Battery or PV cells as in solar as you already would have known or rotating magnet. Migration of ions between +ve/-ve/cathode/anode plates cause dc to flow in battery which consist of chemical and the battery goes flat if there are no more migration. This could be reversed or what is referred to as charge and hence returns the battery to its original potential and stored. In AC,it uses a different method to produce the current. There must be a rotating magnetic field for AC to be produced.That is why the name "Alternating" is there. For it to be produced,there must be motion.Therefore,this cannot be stored as in battery.I hope this helps.

Christian Ohiaeri's answer is most succinct but I'll put forth the most simple answer. Resistance.