Steam Overlay is by default disabled. This is an overly simple fix to enable it.
How to Enable Steam Overlay
In your library, Right-Click on the game and click Properties.
Under Launch Options, enter “–in-process-gpu”.
That’s it. Now, you have access to the Steam Overlay and you can screenshot in-game.
Thanks to b3 for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.