current latest download of LinuxCNC V2.9.8 will not install GRUB on several PC's

Boots and works perfectly in live mode. The same system loads and runs Windows XP with zero faults, and will boot and run an old Mandrake linux installer USB with no errors. (OLD! Like kernel 2.2!) Of course, the NIC doesn't work with Mandrake, but everything installs. It won't boot after because of the lack of UEFI support, but it does install.

Depronman, the reason I jumped in on this thread with you is that I think I might also be having a network installation error. I am wondering if it's relying on having internet access to download any missing packages that it needs, and that it's missing a few critical packages if it can't get internet access.

I installed LinuxCNC 2.9.8 on a virtual machine a few weeks back. It also failed during the grub installation phase. I'm 99% that at least in my case, something with the automatic network configuration did not work properly, and this caused the grub installation to fail. If I recall correctly, the DNS server was incorrectly set, and I had to manually edit /etc/resolv.conf to put the correct DNS server first.

No problem at all I was just a little unsure who Tommy was replying too
I think the grub problem that you are seeing is down to lack of internet
Both myself and friend of mine who knows linux FAR better than I do experienced the same issue with the 2.9.8 iso when trying to install with no internet present

So whilst trying to install you effectively had no internet connection
Same result that myself and friend of mine experienced

My guess is Rufus is the problem, create a new ISO with Balena Etcher

How would Rufus be the problem? If the liveCD version runs, then the ISO has been written properly, and it's easy enough to run the verification check before using it as bootable media.