VNC + LM 19.3 + LCNC2.8 SOLVED

Take this with a grain of salt as i have not used VNC in over 6 years, but if i remember correctly,
This will work:
The remote PC must have the Desktop environment loaded, meaning it needs to have a monitor attached, or have a dummy plug instead of the monitor. Dummy works with VGA, for DVI or HDMI or DP it requires forcing X11 to use a screen when there are none attached.
This will not work:
Remote PC has no monitor or any of the above mentioned. This will force the remote PC to use the local monitor as a screen in software mode, and this will work for most of the stuff, but anything requiring direct access to OpenGL or some other rendering stuff will not work, as can be seen from your error report where X11 can not access the "xrandr" function on the other PC.
Again, none of this is certain, that is all i recall and it is foggy out there.
Can you attach a monitor to the remote PC and try again to connect to the local one via VNC ? Remote PC is the one with LinuxCNC on it controlling the machine.