The only reply by heavy_crown indicates that the file isn't always there and you can feel free to create it yourself. Yes, there's a (dead) link to something that's no longer pertinent. At this point, we might note that Raspbian is basically Debian (as a flavor of Linux) and that link referenced a different flavor called ArchLinux. If you're chasing code on the Internet, it's best to watch out for gotchas like this.
So here you are, following a tutorial or two. You have difficulties with that (noting that you might consider opening an issue on BillyBlaze's repository to give him the head's up about his instructions).
For what it's worth, that
/etc/X11/Xwrapper.config file might be created after you boot once into the Desktop (assuming that you install an HDMI display, mouse and keyboard locally on that Pi). And then—assuming that the file is created after one bootup—that
sed command then should work as you might expect it to.
In the case where you're trying to create it yourself, it's best to look at that
sed command and try to see what it's trying to accomplish. If I search for
sed -i 's/ I see that this is a search-and-replace activity. So it's not going to find anything in a blank file you create; you'd need to create it with some content...
debian /etc/X11/Xwrapper.config and a little light reading gives me this issue with some example content for the file. The first post there is what you'd be after.
sed command from before, the first part is what it would look for and the second part is what it would replace that with. We note that since you're interested in "anybody" as the argument, it would be then unnecessary to run the
sed command, having created the file from the example in the issue post.
So now, how do you create the file in Linux (Debian/Raspbian)?
sudo touch /etc/X11/Xwrapper.config
sudo nano /etc/X11/Xwrapper.config
You're then in an editor. Copy/paste from the example then
Ctl-o as in "oscar", then
Ctl-x to exit the editor.