Hi @ashishk74,
I assume that your crowdsale doesn't have enough/any tokens.
_From: https://docs.openzeppelin.com/contracts/2.x/crowdsales#token-emission_
In the default scenario, your crowdsale must own the tokens that are sold. You can send the crowdsale tokens through a variety of methods
I suggest having a look at: Simple ERC20 Crowdsale.