You need to manually create the bundle for a debug build.
Bundle debug build:
#React-Native 0.59
react-native bundle --dev false --platform android --entry-file index.js --bundle-output ./android/app/src/main/assets/index.android.bundle --assets-dest ./android/app/src/main/res
#React-Native 0.49.0+
react-native bundle --dev false --platform android --entry-file index.js --bundle-output ./android/app/build/intermediates/assets/debug/index.android.bundle --assets-dest ./android/app/build/intermediates/res/merged/debug
#React-Native 0-0.49.0
react-native bundle --dev false --platform android --entry-file index.android.js --bundle-output ./android/app/build/intermediates/assets/debug/index.android.bundle --assets-dest ./android/app/build/intermediates/res/merged/debug
Then to build the APK’s after bundling:
$ cd android
#Create debug build:
$ ./gradlew assembleDebug
#Create release build:
$ ./gradlew assembleRelease #Generated `apk` will be located at `android/app/build/outputs/apk`
P.S. Another approach might be to modify gradle scripts.