Thought I would add that I am using Unity 5.6.0f3, JDK 1.8.0 and Android SDK 25.2.3.
Can you please check if the application has camera permission enabled on the device? Have you tried running on another device as well?
Alexandru from Tech Support had delivered a workaround here: https://support.wikitude.com/support/discussions/topics/5000083773/page/2?url_locale=
Disable "Multithreaded Rendering" from Player Settings -> Other Settings tab.
I downloaded the SDK and have built the Client Recognition Instant Tracking sample scene to a Samsung S6 without any changes. The application builds and runs on the device, and I am able to instantiate furniture, move and resize, etc. However, there is no display from the camera... the background is white. I've tried enabling the Camera2 API but this has no effect.