Getting an error: Intsallation failed: Configuration Error.
I need some help please.

Are you installing the game through GOG Galaxy? Try to use the offline installers which don't require the client.

Sarafan, thanks for the reply. Yes, I'm using GOG Galaxy. you mention offline installers. Where would those be. I apologize, I'm pretty lo-tech. Appreciate the help! :)

Move your mouse cursor on your nick at the top of web browser window, choose Games, find Witcher 3 GOTY, click on the "Download offline backup game installers" and download all of the necessary files to one folder. Then go to that folder and launch the .exe file. You can also use GOG Galaxy to download offline installers. To do so, choose library, find Witcher 3 GOTY, click on it, click on More and choose Backups & Goodies. Now download the installers. They will download to a folder selected in the settings of GOG Galaxy.

Thank you, my friend! :)