I have installed create-react-app exactly as instructed on the facebook instruction page (https://facebook.github.io/react/blog/2016/07/22/create-apps-with-no-configuration.
Try this. It worked or me. I found this in the React documentation. "Npx" is not a typo. It's a package runner tool that comes with npm 5.2+.
npx create-react-app my-app