Try these basic troubleshooting steps:
Check that anything (case or screen protector) is obstructing or interfering with your front camera.
Force restart your iP6s: press and hold both Side (power) and Home buttons for 10-15 seconds, until youe see the Apple logo. When it appears, release both buttons, wait for your iphone to restart, then test it.
Read and follow the instructions provided in this support guide:
Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support
If your front camera issue still persists, install a fresh copy of your iOS 11.4.1 (it could be corrupted).
Make a full backup of your device's content, then restore it to factory settings and, at the end, set it up as new.
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
When done, test your iphone for a while, WITHOUT restoring your data from the backup. If your camera works fine, you can exclude a hardware failure.
Then restore your data from the previously made backup:
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
If the issue still persists, make an appointment at your most convenient Apple Store or Authorized Service Providere and have your iP6s inspected. Apple - Find Locations - Location Selector
Regards
Giulio