Front flash. Iphone 6s plus

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.

Hi

To use Retina Flash with the front-facing FaceTime camera on your iPhone 6s Plus:

• Open the Camera app.
• Turn on the flash.

  Tap the Flash / lightning bolt icon to access the flash options.

• Switch to the front-facing camera.
• Tap the Shutter button.

  The display flashes on exposure.

You have since confirmed (via a duplicate thread), that you have an iPhone 6 Plus, with model number A1524.

Retina Flash (for the front-facing camera) is not supported on your iPhone model and there is no way to enable it.

There is no "front flash". Only the rear or main cameras on iPhones have a flash.