MaxPro (Mid 2012) OS upgrade

once you have MacOS 10.11 El Capitan fully updated, you can upgrade readily to 10.13 High Sierra.

How to download and install macOS - Apple Support

But to then move on to 10.14 Mojave, you need to take that step staring from 10.13.6 High Sierra, AND you must have a Metal-compatible graphics card installed, or it will refuse to install.

Install macOS 10.14 Mojave on Mac Pro (Mid 2010) and Mac Pro (Mid 2012) - Apple Support

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(I am assuming you are using a Mac Pro (as you said) and not a MacBook Pro)

The Mac Pro (Mid 2012) came with Mac OS X 10.7.3 (Lion), you are running 10.11 (El Capitan), and the latest you can upgrade to is 10.14.7 (Mojave) and can be upgraded to 10.14.7.

To upgrade to 10.14.7, you need to visit

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

https://apps.apple.com/us/app/macos-mojave/id1398502828?ls=1&mt=12

or

macappstore://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12

... (make sure you do this from Safari - not Chrome.)

It's a good idea to consider using Time Machine in conjunction with your external hard drive for continuous data backups. Instead of manually dragging selected files to a backup drive, Time Machine offers a more comprehensive solution. If the need ever arises to restore and reinstall everything, Time Machine will prove to be an invaluable tool for that purpose.