F12 does nothing and I can't remap to another key as screenshot is not showing up in Oblivion keys.
Using GoG Galaxy (the new purple one).
I'm not technical and my eyes are bleeding (slight exagerration) from trying to understand other posts about this.
Can some kind soul tell me an easy (for idiots) to follow solution?
Please help.

If you're using Windows, pressing the Windows key (the one at the bottom of the keyboard with the Windows logo) + PrtScn (Print Screen) usually works. It automatically saves a screenshot of what you're doing in Pictures ---> Screenshots folder.