My iPhone 6 plus mobile rear camera is not working properly

It's difficult to diagnose that kind of issue remotely, but it could be a hardware failure.

As suggested on my previous post, have your device inspected at your most convenient Apple Store.

My best regards.

Giulio

Read this support guide and follow all the steps provided:

Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support

If you have no improvements, I would contact your most convenient Apple Store or Authorized Service Provider and have your iPhone inspected.

See this list: Apple - Find Locations - Location Selector

Regards

Giulio