Tried every net solution for days to install linuxcnc

What image or installer at
"www.debian.org/devel/debian-installer/" should I try? Best fit for my system?

The top left one.
Netinst CD image for AMD64

After Disable secure boot and enable legacy, I got a new error screen with similar message, about not finding the the boot disk. The USB drive is good.

"HDD? Perhaps its a legacy device" I think it is a legacy device with “UEFI: IPv4 Realtek PCIe FE family controller” and “UEFI: IPv6 Realtek PCIe FE family controller” on it somehow. I did reformat it a few times during the partitioning set up. Not sure how "Realtek" keeps coming back unless it is hiding somewhere else or remaining after reformatting.

My USB hard drive is much newer and I know it is good, so I am trying everything I can think of to get linuxcnc on this system. The "boot loader" I believe is in the ISO with everything else it needs to install and I did load it onto both internal and external USB drives. I did try to sign up to the Debian forum, but got the violin questions backwards, now they think I am a robot AI and won't let me join for a month.

I will try the ISO you suggest. The one I did try is "debian-12.9.0-amd64-netinst.iso" and using "rufus-4.6_x86.exe" to make a bootable image for USB thumb drive and USB hard drive.

At the end of the installation when it asks you where you want to install the boot loader it should be installed onto the drive that Linux is installed to.

I believe your biggest hurdle is trying to install to a USB drive. The simplest way to install Linux is to a drive that is connected to a SATA or IDE port, or NVME if you have one.

"At the end of the installation when it asks you where you want to install the boot loader" None of the installations that "installed successfully" asked anything about a "boot loader" or where I wanted it to be. I did run across some suggestions that it was left out of the installation repository somehow. I would think it was part of the ISO since it was required for a successful installation.
"The simplest way to install Linux is to a drive that is connected to a SATA or IDE port, or NVME if you have one" The first thing I tried was to install the ISO to the internal hard drive with the suggested partition, formatting, etc. set up for that install.

Well if it didn't, at least for the Debian install something has gone terribly wrong.
Without the installation of a boot loader, or where you want to install grub, the installation won't boot.
But some of this is starting to sound like a troll, based on your previous answers as to where you installed the boot loader. And the fact you never mentioned you tried installing to an internal hard drive to begin with.