Error on Android app starting - Error running app: Default Activity not found

This is because you might not have added intent-filter to any of your Activities in AndroidManifest.xml
Add intent-filter to your Starting activity, if your starting activity's name is MainActivity then the entry looks like below

<activity android:name=".MainActivity">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>