1- Make sure to create a local.properties inside /android directory
2- Then write this sdk.dir = /Users/<username>/Library/Android/sdk (replace <username> with your username)
if you run somehow through this error: facebook/react-native#7308
Run this cmd: killall -9 node to kill any node port running.
If you run somehow through this error: unable to load script from assets index.android.bundle
Run this cmd: 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