The above linked writeup simply assumes that there won’t be a GUI session at the Manjaro machine where the vncserver will be installed, perhaps true enough for some Raspberry Pi sitting somewhere.
Otherwise, if to a Manjaro installed on a desktop and having a GUI session, your VNC connection to it will fail (at least if the user is the same for both the GUI and VNC).
To avoid GUI’s preempting VNC: At bootup of Manjaro, use Ctrl + Alt + F2 to log into a terminal session and start vncserver’s systemd service:
$ sudo systemctl start vncserver@:3
Or you can enable the service (so it starts automatically with boot) and not log in. But in this case, you have to stop the service (in a terminal session) first before you can log in for a GUI session.
$ sudo systemctl stop vncserver@:3
Second point, not everybody wants SSH. Some machines are confined to LAN. In this case, just don’t enter localhost to config. Let us not complicate matters beyond what is necessary.
Third. ls /usr/share/xsessions will give you a return like fxce.desktop. The value you want to put into config is just xfce, without .desktop. That wasn’t clear.
For background, you can see these. (I don’t believe they have extra info.)
Conversely, trying to log into a local X session while a VNC server service is running for that user will likely not work, and you may get stuck on a splash screen when using a desktop environment.
– TigerVNC - ArchWiki
I did link you to an alternative approach in the original topic which - albeit for Plasma - deals with running a vnc service connected to screen :0