Retina Flash is available on iPhone 6s Plus (which has the model number A1634, A1687 or A1699 on the back), but not on iPhone 6 Plus. The flash feature uses a white flash of the display when taking the photo.
If you have an iPhone 6s Plus and you have not yet done so, it may help to update your iPhone to the latest version of iOS:
Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
Be sure to make a backup first:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Also check that your Camera app is in "Photo" mode.