I had the same problem. Following command worked for me:

After the ./gradlew bundleRelease command we get a .aab version of our app. To 
get APK, you should run the app with release version on any device with the 
below command.

Make sure you have connected an android device
For the production ready app, firstly you have to remove the previous app from
the device. Run this command in your-project/:

react-native run-android --variant=release

Then APK can be found in android/app/build/outputs/apk/release. Hope this helps