It's difficult to convert a React web app into a React Native app, but you can use many React Native components on the web (via react-native-web. [1]) In other words, you can't run a web app on mobile, but you can run a mobile app on the web.

I just launched a little game that I built with React Native [2], where I have a single codebase that supports iOS, Android, Windows, and web.

[1] https://github.com/necolas/react-native-web

[2] https://sudoblock.com