I took some lovely pics of 56 Sqd SE5 on the way to the front with amazing sun and sky to post here, only to find none of them where saved to the PC!! Probably due to #%&*$# Avast.
I use Greenshot for all my screenshots, in game and out. I also use Avast and have never had a problem with Greenshot. If you install it be sure to go through the Preferences to set it up the way you want it.
K doesn't do it for me either. Dunno why. Don't use avast. Couldn't be bothered finding out why cause that #%&*$# can send you insane. So I use the nvidia gforce thing for in-game and prtscn (paste into paint) for GUI shots. So it's no big deal. Kiss the K key goodbye and kick it down the road.
Or..use reshade and you dont have to have "inject" any tweaks but you can set it to use Printscreen key to capture shots and also have a nice handy fps counter in the corner if you so wish
They shall grow not old, as we that are left grow old: Age shall not weary them, nor the years condemn. At the going down of the sun and in the morning We will remember them.