AR Image Tracking problem....Camera not working

I am trying to do AR image tracking.

But when I launch to a handheld device the camera is black (I can’t view anything). This happens when I assign a Candidate Image to the Data Asset ARConfig. I also tried creating a creating a Augmented image Database and I got the same result. When I remove Candiate Image or the Augmented image database. The camera works fine. But then I can not use image tracking for my target.

Can some one help me with this issue?