How to reproduce:
1. Open SDK Manager, delete all newest API and make sure that you download API 19, 20 and/or 21
2. Start a new project in Unity and save the scene
3. Switch platform to Android in Build settings (don't forget to add scene to build)
4. Change Bundle Identifier in Players settings
5. Set Minimun API level and Target API level to 19/20/21
6. Check Virtual Reality Support and add Cardboard
7. Build and Run
Result:
Warning: [Temp\StagingArea\AndroidManifest-main.xml:12,
D:\Proyectos\Unity\prueba2\Temp\StagingArea\android-libraries\unitygvr\Andro
idManifest.xml:3] Main manifest has <uses-sdk android:targetSdkVersion='19'>
but library uses targetSdkVersion='24'
]
exit code: 1

Note: According to documentation https://developers.google.com/vr/unity/get-started for Cardboard it is enough to have 19 API ;
If one sets Target API Level to Automatic, build fails but error is not so clear.

Reproducible: 5.6.0b3, 5.6.0b9, (5.5 didn't have a possibility to enable Cardboard SDK)