Failed to load AppCompat ActionBar with unknown error in android studio

The solution to this problem depends on the version of the Android support library you're using:

Support library `26.0.0-beta2`

This android support library version has a bug causing the mentioned problem

In your Gradle build file use:

compile 'com.android.support:appcompat-v7:26.0.0'

with:

buildToolsVersion '26.0.0'

and

classpath 'com.android.tools.build:gradle:3.0.0-alpha8'

everything should work fine now.

These new versions seem to suffer from similar difficulties again.

In your res/values/styles.xml modify the AppTheme style from

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">

<style name="AppTheme" parent="Base.Theme.AppCompat.Light.DarkActionBar">

(note the added Base.)

Or alternatively downgrade the library until the problem is fixed:

implementation 'com.android.support:appcompat-v7:28.0.0-alpha1'

found it on this site, it works on me. Modify /res/values/styles.xml from:

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
</style>

to:

<style name="AppTheme" parent="Base.Theme.AppCompat.Light.DarkActionBar">
</style>

June 2018 Issue fixed by using a different appcompact version. Use these codes onto your project dependencies...

In build.gradle(Module: app) add this dependency

implementation 'com.android.support:appcompat-v7:28.0.0-alpha1'

Happy Coding... :)