In the event that you are attempting to begin a Darkroom photo session and after a few picture taking try's, a "Session Fail" error message comes up, simply open the back of the booth that houses the camera and adjust the camera mount by the small silver screw at the bottom (pictured below).  By loosening this screw on the camera mount, you can either push the camera slightly forward toward the Mirror glass or backwards, slightly away from the Mirror glass.  This process always corrects the problem because the slight distance between the lens and the Mirror glass dictates how the camera auto focuses. So, by slightly adjusting the camera, you will have corrected the issue.