Hi @Revinand,
Welcome to the community 
.
Crowdsales have been removed from the OpenZeppelin Contracts v3.0 beta release and there are no plans to migrate them to Solidity 0.6. If you need a crowdsale you can use OpenZeppelin Contracts 2.5:
Crowdsales were removed: we’ll continue to provide support for security issues on the v2.5 release, but will not bring them over to v3.0.
Note: You should only use code published in an official release of OpenZeppelin Contracts. When importing via GitHub on Remix you can specify the release tag, (otherwise you will get the latest code in the master branch).