The iPhone 6Plus units that were sold between September 2014 and January 2015 had a problem that resulted in camera failure for many users. Apple acknowledged the problem and launched an iSight Camera Replacement Program for the iPhone 6 plus devices. The problem affected a limited number of devices only. It is said that the OIS feature in the camera could be the possible reason for the failure because the iPhone 6 was not affected by it.
Solution: Request a Replacement for Your Phone If It Is Under Warranty
If your phone is under warranty and you are experiencing problems related to the camera, then take it to the Apple store since you may be able to get a replacement. The problem is difficult to fix and it is not a software issue.