How to add a WatchOS SWIFT Target to an iOS app written in Objective-C

I am trying to add a basic WatchOS Target to the "Hello World" React Native iOS app. It seems like the React Native iOS App is written in Objective-C, but I want to use Swift in my WatchOS app. If I select Objective-C when adding the WatchOS Target, everything works. But if I select Swift using the WatchOS Target, I get the following error in my WatchApp Extension:

Command CompileSwiftSources failed with nonzero exit code

Is there a way to add a Swift WatchOS Target to a React Native iOS app, or any other app written in Objective-C?