React Native app crash: Unable to load script from assets 'index.android.bundle'

You should try to downgrade gradle build tool from 3.2.1 to 3.1.4

Downgrading from gradle build tool 3.2.1 to 3.1.4 solved the issue for me too. It took me a couple of days to figure it out.

You don't specify which version of react-native you're using. I had the same issue with 0.57.3, and the other answers put me on the right track: there was a bug in the android build system with android gradle plugin version 3.2, which was solved in 0.57.5 (changelog).

The bugfix is here and is very simple to manually integrate if you don't want to upgrade RN.

The fix is upgrading RN to 0.57.5 or downgrading gradle to 3.1.4.

For those who want a really neat workaround without having to upgrade/downgrade:

In your app/build.gradle add jsBundleDirRelease:

project.ext.react = [
    entryFile: "index.js",
    jsBundleDirRelease: "$buildDir/intermediates/merged_assets/release/mergeReleaseAssets/out"
]