If you are behind a proxy, set it correctly in npm.
npm config set proxy http://proxyhostname:proxyport
npm config set https-proxy http://proxyhostname:proxyport
Try to disable ssl and set registry to http instead of https.
npm config set registry=http://registry.npmjs.org/
npm config set strict-ssl false
Note1: For SSL/https proxies the protocol in URL should be http not https.
Note2: If your set up is on Vagrant instance or a hosted VM use IP instead of hostname for proxy as it might not be resolvable.
If there is no proxy , remove proxy config from npm
npm config set proxy null
npm config set https-proxy null