LinuxCNC Error

I have 2.9.3 with the 5.4.258 RTAI kernal and am getting an error when starting my mill program.

     insmod: ERROR: could not load module /usr/realtime-5.4.258-rtai-amd64/modules/rtai_hal.ko: no such file or directory 

Do you also have the RTAI version of LinuxCNC? It seems you do as the .ko should be RTAI and .so should be Preempt.
Paste the full error report here, please.

After I got the error and copied that specific part the error changed it won't come back again.

How would I get an RTAI version of linuxcnc, I have been looking at the getting started and installing linuxcnc and haven't seen anything like that.

I am so confused right now because the documentation has instructions but it seems like packages are missing from the pre-made files, and the documentation that is stated does not work. Then when I go to forums I am being told that some things may be missing. I am so lost right now, I don't know what to follow and what to ignore, out of date info, what files do and don't work with each other...etc.

Normally after installing the 2.9.3 ISO, you should do
sudo apt purge linuxcnc-uspace
sudo apt install linuxcnc
then after reboot hold down the shift keys till you get the GRUB menu, there choose advanced options and select the one with RTAI in the name, enter.
Did you do this last step before testing?

I never seen any documentation saying to use those commands in that way. I will try that and get back to you.

when I run "sudo apt install linuxcnc" it returns

Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Package linuxcnc is not available, but is referred to by another package.
This may mean that the package is missing, has ben obsoleted, or is
only available from another source

E: Package 'linuxcnc' has no installation candidate