(superseeded) Linuxcnc on Linux Mint 19.3 quick and easy installation

You did not hold down CTRL+Shift when booting the computer.

no i did not i will try now

OK

i still got the same reply

Did you get the GRUB menu?
You have to chose the other kernel there.

In a terminal:

sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer

Start grub-customizer from the menu, select the 4.14.28-rt23-kona-rt and use the UP arrow to move it to the top of the list, save, exit.
Reboot.