Assuming you installed XAMPP correctly, there’s a strong possibility you didn’t install RPints in the Root Directory and why the config can’t be found.
I’ll be honest, the tutorial you referenced is pretty decent and I can’t offer much else.
Typically setting up the web server and a root directory is the toughest part.
I did this years ago, setting up RPints on an old Windows Desktop (heck, it’s still running). I am now on an RPi running RandR+’s Fork, and happier for it. IMO...you’re on a learning curve no matter how you go about RPints...I see value (and sanity) going a route where more assistance is available.