why will it not load on my mac pro desk top

So to confirm, you tried both of the sequences listed above, and both failed to allow the Mojave installer to launch?

Mac Pro 2012 is supported on Mojave, with a Metal-capable graphics controller.

How to upgrade to macOS Mojave - Apple Support

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

Delete the Mojave installer (and you might have more than one copy around), re-download it, and try the install again.

Your crystal ball must be a newer model than mine ; )

You have a download containing the Mojave installer. You’ve tried launching it. Before the installer starts, you get a message indicating that the download is corrupt.

Two paths:

First: Find all existing copies of Mojave installer, delete them, and re-download. Try launching the re-downloaded Mojave installer.

Failing that, second: If the freshly-downloaded Mojave installer fails to launch, launch System Preferences and set your system date to manual and to sometime in September 2018, re-launch the Mojave installer, and when you get to the first prompt in the Mojave installer, pause and go back to the time and date settings in System Preferences, and reset the date and time to automatic, and to the current date and time. Then return to and continue with the Mojave installer.

Please help us to help you by telling us what won't load on what model/year desktop.