help

See the following from Identify your Mac Pro model - Apple Support

Mac Pro (Mid 2012)

...

Newest compatible operating system: macOS Mojave 10.14.6.*

* To install macOS Mojave on Mac Pro models from 2012, a Metal-capable graphics card is required.

Mojave is the highest you can go: your Mac (which is pretty old) doesn't have the hardware requirements for Big Sir or indeed for Catalina.