React Native modules that include native Objective-C, Swift, Java, or Kotlin code have to be "linked" so that the compiler knows to include them in the app.
For iOS, while you can manually link the old way using [react-native own tutorial](https://facebook.github.io/react-native/docs/linking-libraries-ios), we find it easier to use cocoapods.
If you wish to use cocoapods and haven't set it up yet, please instead refer to [that article](https://engineering.brigad.co/demystifying-react-native-modules-linking-ae6c017a6b4a).
For Android manual installation, please refer to [this article](https://engineering.brigad.co/demystifying-react-native-modules-linking-964399ec731b) where you can find detailed step on how to link any react-native project.
Cocoapod and autolinking is not yet support for react-native macOS but is coming soon. In the meantime you must manually link.
The method is nearly identical to the [manual linking method for iOS](https://facebook.github.io/react-native/docs/linking-libraries-ios#manual-linking) except that you will include the `node_modules/react-native-webview/macos/RNCWebView.xcodeproj` project in your main project and link the `RNCWebView-macOS.a` library.