No apps connected. Sending “reload” to all React Native apps failed. Make sure your app is running in the simulator or on a phone connected via USB

⚠️ For iOS users :

Check that your build target is Debug :

Xcode Product => Scheme => Edit Scheme => Debug (and not release ! )

⚠️ For Android users :

I have found a solution that I posted on github.
Not the best but it could be useful:

First, you have to open MainApplication.java
and remove import com.facebook.react.BuildConfig;

Next, follow these steps :

    # Reset metro bundler cache : 
    `npx react-native start --reset-cache`
        
    # Remove Android assets cache : 
    `cd android && ./gradlew clean`
        
    # Relaunch metro server : 
    `npx react-native run-android`

And see the magic 🎉

See here :
https://github.com/facebook/react-native/issues/29396