LinuxCNC on Linux Mint 20 quick and nearly easy install

I will reinstall it.

Maybe a graphic card solve this?

Its the same.... already try to reinstall 3 or 4 times...

without the rt it works:

mjg@CNC:~$ xrandr
Screen 0: minimum 320 x 200, current 1024 x 768, maximum 16384 x 16384
DP-1 connected primary 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
1024x768 60.00*
800x600 60.32 56.25
848x480 60.00
640x480 59.94
HDMI-1 disconnected (normal left inverted right x axis y axis)
mjg@CNC:~$

Try another graphic card, graphic card way to old or .....

It's onboard....

Tomorow have a new one

I followed the instructions and it mostly worked for me. The kernel referenced doesn't seem to exist in that repository and it's kinda old now anyway so I installed 'SMP PREEMPT_RT Debian 5.10.13-1~bpo10+1 (2021-02-11)' from the same repository. I can report LinuxCNC works under that kernel on Mint 20.

I did struggle with signing the kernel for Secure Boot, never used that before. After spending hours following several instruction sources several times, trying both signed and unsigned kernel packages, I lost patience and disabled Secure Boot in Linux.

Other than those couple of issues it was rather straightforward following your instructions. I'll be hooking up a Mesa 7i76e, which I've also never done previously.

wget ftp.us.debian.org/debian/pool/main/l/lin..._4.9.228-1_amd64.deb
That still works, and that is the one used in the tutorial. It was 5.4 but that is not found any more so glad you found 5.10.
Secure boot is another microsoft "showed upon us" useless feature.