Our story begins with me buying a refurbished Mac Pro in 2009. The next significant develop in the computer's story is macOs 10.13 (High Sierra).
The 2009 Mac Pro (4,1 if you're into being specific) won't update past 10.12 (without doing something that Apple doesn't condone to it).
The other annoying thing is that it won't boot off of any of the Linux test drives I made. This has to do with the 32 bit EFI implementation. I'm not an expert on this part; there's lots to read about it on the web if you want to go down that rabbit hole.
Tricks exist, including modifying the installer, modifying the firmware, etc. Other people offer you pre-built solutions, with warnings that you shouldn't be installing other people's modified distros right in the page.
A simpler trick - pull the harddrive and install Linux using something that has modern firmware. Then pull out all the other drives from the Mac Pro, stick the drive in and wait for it to boot up in the new OS.
There's lots that's out of date with that machine, don't get me wrong. But it's ridiculous to not be able to use it anymore for the things it still is good at - it's got 8 cores, tons of ram... and it can stay on for days at a time, with no battery charging, or getting stuffed in a bag, or losing network connectivity.
Lots of hardware, in fact, gets thrown away that could be put to some other use, and that's a shame - it drives us to buy more new computers, create waste in the manufacturing process. From an ecological standpoint, this machine uses more electricity than a new Intel NUC, but we've already paid the manufacturing price - and it's cheaper that way, many times over.