Question: I'm new in react-native.
I have run react native project on Ubuntu by using 'react-native run-android' command. And I got the error on emulator 
"Unable to load script from assets 'index.android.bundle'.Make sure your bundle is packaged correctly or you are running a package server."

Answer: 
In my case (embedding React Native as a new `Activity` into an existing Android Code Base), the problem was Android Studio had auto-imported the wrong **BuildConfig**.

Wrong:
`import com.facebook.react.BuildConfig;`

Right:
`import com.mywebdomain.myapp.BuildConfig;`

This would apply to the wherever you are housing this block of code:

```
mReactRootView = new ReactRootView(this);
mReactInstanceManager = ReactInstanceManager.builder()
        .setApplication(getApplication())
        .setBundleAssetName("index.android.bundle")
        .setJSMainModulePath("index")
        .addPackage(new MainReactPackage())
        .setUseDeveloperSupport(BuildConfig.DEBUG)
        .setInitialLifecycleState(LifecycleState.RESUMED)
        .build();

```
In my case the problem was in this row in the React Activity file:

```
mReactInstanceManager = ReactInstanceManager.builder()
...
   .setUseDeveloperSupport(BuildConfig.DEBUG)
...

```

BuildConfig.DEBUG must be set to true, while in my case it was false