Mac OSX Last Possible Version
Hello, what’s the last version one can install on a Mac Pro 5.1 2009 model please…?
Mac Pro
The last supported operating system is macOS El Capitan (10.11.6). See How to get old versions of macOS - Apple Support. Using Safari, you can download El Capitan from the Mac App Store by following the link on that site. Read carefully, the paragraph preceding that El Capitan link so you will know how to access the installer.
Take a Time Machine backup prior to any upgrade, and do not reuse that Time Machine drive for the new operating system backup.