How to install Linuxcnc. . .

  If the Linuxcnc installation screen does not appear, what is the reason and solution for the attached picture?

in PC BIOS:
disable UEFI boot
enable Legacy boot
disable Secure boot
retsart

That said, I bought a UEFI PC specifically to test the LiveCD, and it worked for me.

I am assuming that your problem is that "Start Installer" does not start the installer?

I think there is a non-graphical installer under "Advanced options". You could try that.

UEFI is very finicky, i can install Mint 20 on all UEFI boards with it active but i can not install it on a HP laptop/tablet that has UEFI and it can not be disabled. Mint 19 installs on that same laptop/tablet but does not on all other boards i have. All boards are B550, X570, X470 and B450 made by Asus and Gigabyte and MSI, all with their latest respective BIOS. Intel got the boot !
Both Mint versions install on Dell 9020 and 7010 though with UEFI and without it.

Somehow the installation was successful, but the uefi problem on the laptop or on a specific board is quite annoying.