In some cases adding a package via yarn will result in iOS Pods being automatically installed. When this happens the Pod React-Codegen has it's iOS Deployment Target rest to 11.0 causing builds to fail.
To fix this open Xcode, click on Pods in Project Navigator, and look for React-Codegen under targets. Scroll down to the property iOS Deployment Target and change to 12.4.
If Android changes are not detected when debugging, run the following command.
npx react-native bundle --platform android --dev false --entry-file index.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res/
source ~/.bash_profile
adb reverse tcp:8081 tcp:8081
npx react-native run-android
However, after running that command several folders are created that cause the command ./gradlew bundleRelease to fail. Prior to running this command delete the folders in \android\app\src\main\res that start with \drawable-, being sure to KEEP the folder named drawable.