RPI4 Raspbian 64 bit & LinuxCNC

From what I see I think it's a problem due to updating.
I recommend, before installing the software dependencies, to run these commands:

sudo apt update
sudo apt upgrade

Thank you for your input, this did indeed solve the issue.

Success!
After floundering on-and-off for years (no joke) trying to get a functioning install, this thread hands me a functional solution on a silver platter ! I've somehow always managed to find myself with some sort of hardware compatibly issues - this time, just as warned, the official SD image doesn't play nicely with my Pi4 8GB . But (!), as I've already said, the image supplied by elovalvo (on page 16) is showing more signs of life than anything I've managed to procure on my own:
image missing https://i.imgur.com/5FfsCRX.jpg
I have a some ways to go to get it to work with my Mesa 7C81, but I needed to make an account to say THANK YOU to elovalvo ~

There is also the Debian way. I'm hoping this will eventually be accepted into Linuxcnc docs
vmnet.com.au/linuxcnc/getting-linuxcnc.html

This is now superceded as its in the 2.9 documents now.
Please refer to this link instead. linuxcnc.org/docs/2.9/html/getting-start...etting-linuxcnc.html

I have used your 2.9 version. Running successfully in lathe.
For further enhancement I have used your latest book.zip.
Running smoother than previous one.
But in gmocappy side and lower menu not showing.

Thats more likely tto be a monitor size issue. Take it up in the Gmocappy section.