I had almost the same problem you currently have a few days ago. For me it was a real device.

From all my research, the solution that worked for me is the following :

When you launch your app by typing react-native run-android, the error appears.

So, reach your directory containing "adb.exe" ( for me it was C:\Users\username\AppData\Local\Android\Sdk\platform-tools\ )

open a terminal from here. And type adb reverse tcp:8081 tcp:8081

Then, by reloading the app on the phone, it should work.

I hope this will help you !! ( if not, shame on me )

Cheers ;)

EDIT :

I've find a more convenient solution to solve this problem.

When you launch your "react-native run-android" you might have a comment saying "adb is not recognized as internal or external command..."

Well, instead of manually using the adb command, you can add an environment variable leading to the adb.exe.

Just get to the "Control Panel" > "System and Security" > "System" > "Advanced System Settings".

From here, click on "Environment Variables". Search for the "Path" variable in the "System Variables" section. Click on it and press the "Edit" button.

There you can add a new path to this variable. Just click the "New" button and write the adb.exe path. For exemple C:\Users\username\AppData\Local\Android\Sdk\platform-tools\

(For the oldest versions of Windows, you'll have to add ";C:\Users\username\AppData\Local\Android\Sdk\platform-tools\" to the content.)

This is it, now the adb reverse tcp:8081 tcp:8081 command will be ran automatically upon building.

Hope this will even more helps you !

I had the same issue, my fix was to run

$ react-native start

in the background then run

$ react-native run-android

According to me this error comes when:

1) You did not run the packager at the right place i.e, you hit the wrong folder to run react-native packager so just navigate to your project's directory and run react-native start.

2) when you code something wrong with respect to Appregistry module.

3) Shutdown everything Xcode, packager, simulator etc and restart it again.

4) From the error one thing strikes in my mind is the name of the module

AppRegistry.registerComponent('AppName', () => componentName)

matches with the @"AppName" on your AppDelegate.m on the call for

[[RCTRootView alloc] initWithBundleUrl:...
                            moduleName:@"AppName"
                         launchOptions:...

5) As we are using react's component so we need to give the first character of our class name with caps on.

6) Let's see what's wrong in index.android.js and index.ios.js

Cheers :)