I realize a 2012 Mac Pro is rather senior for a piece of hardware, but what it is the most recent version of High Sierra that is available? I currently have it up to MacOS 10.13.6.
Latest MacOS for a 2012 Mac Pro
I will assume you meant the most recent macOS version for this Mac Pro, and not the most recent High Sierra version.
This Mac tops out at Mojave, with a Metal-capable graphics controller installed.