Hi Aremu,
Assuming you've tried all of the possible software solutions to no avail, then you're left with the distinct possibility you've got a hardware problem with your phone. You have tried the last resort software fix of doing a complete wipe and restore of the phone, right?
Assuming your camera and flash still aren't working after that, there are two possible parts that could have failed; either the rear camera has died or the flash LED itself has; it's entirely possible that one of those two components not working could affect the other. I'm not sure how to diagnose one part versus the other, outside of perhaps opening up the phone and disconnecting each of those parts in turn then seeing if the other part works after that.
Here's the plug for the audio/flash/microphone assembly you'd need to disconnect. Unplug it then test the camera and see if it's working; if so you'll probably need to replace this assembly.
This picture shows the rear camera connector. If you unplug it and the flash works, you'll probably need to replace the camera.
Replacing the camera is relatively straight-forward once you have the phone opened and the screen removed; A quick Amazon search shows the replacement part available for about $11 USD.
Amazon.com: MMOBIEL Main Rear Back Camera Replacement Compatible with iPhone 6s 2015 12 MP - Incl. Screwdrivers : Cell Phones & Accessories
Here's the iFixit guide for replacing the camera.
iPhone 6s Rear Camera Replacement - iFixit Repair Guide
The audio/flash/microphone assembly is a much more difficult repair, unfortunately; you have to remove a significant number of parts internal to the phone to get to it, so it's not a repair for the faint of heart. That part is also available on Amazon for about $11 USD.
Amazon.com: Johncase New OEM Power Volume Button Control Switch On/Off + Flash Light + Mic + Mute Connector Flex Cable w/Bracket Replacement Parts Compatible for iPhone 6s (All Carriers) : Cell Phones & Accessories
Here's the repair guide you'll need to do the job.
iPhone 6s Upper Component Cable Replacement - iFixit Repair Guide
Should you decide to replace both, you can just use this guide, as removing the rear camera is one of the steps required to replace the cable, so you can do both in one operation if desired.
That's about all I can come up with for you; there are some actual repair technicians who frequent this site and often offer their experience, so hopefully one of them may be able to offer you some alternatives.
Good luck; let us know how it goes!