What platform are you testing react-native-webview on? My guess is that you’re testing inside the editor, which means that you’re on web. As far as I know, react-native-webview is only iOS and Android compatible.
Instead, on web you can use React code, and then have a separate version for mobile. Alternatively, this library implements react-native-webview for web. If you use that library, just don’t do the aliasing that they suggest in the readme.