If the above error occurs, first confirm whether the RN service runs successfully

In some cases, the startup fails, or it is shut down unexpectedly after startup, or the ports are inconsistent.

Another situation is recorded here:

If it is a project from the new cluster on GitHub, the directory of Android\app\SRC\main\assets may be empty (the assets folder needs to be created). At this time, it needs to be run under the terminal of the project root directory:

react-native bundle --platform android --dev false --entry-file index.android.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res/ 

To create the missing file

After running, you will see the missing index on the red screen android. Bundle file.

Run it again and the problem is solved.