Unable to load script.Make sure you are either running a Metro server or that your bundle 'index.android.bundle' is packaged correctly for release

Question

You haven't started the bundler yet. Run npm start or react-native start in the root directory of your project before react-native run-android.

These steps really help me:

Step 1: Create a directory in android/app/src/main/assets

Linux command: mkdir android/app/src/main/assets

Step 2: Rename index.android.js (in root directory) to index.js (Maybe there is an index.js file in which case you do not need to rename it) then run the following command:

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

Step 3: Build your APK: react-native run-android

Please use index.js in latest version.

Enjoy :)

For me this error was caused by an upgrade of react-native

Starting with Android 9.0 (API level 28), cleartext support is disabled by default.

If you check out the upgrade diff you need to create a debug manifest android/app/src/debug/AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">
    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>
    <application android:usesCleartextTraffic="true" tools:targetApi="28" tools:ignore="GoogleAppIndexingWarning" />
</manifest>

See for more info: https://stackoverflow.com/a/50834600/1713216

https://react-native-community.github.io/upgrade-helper/

A similar issue happened with me.
Apparently Mcafee blocks 8081 port. Took me hours to figure this out.

Try running:

react-native run-android --port=1234

When app shows up with an error on emulator, get into dev settings (Ctrl+M).

Change the "Debug server host and port for device" to "localhost:1234".

Close the app and start it from the app drawer.