my macpro mid 2012 will not download latest software update...shows error in dowloading

If you mean Catalina 10.15, you can't... officially...

Which devices support macOS Catalina?

Apple has gone to great lengths to ensure that macOS Catalina works well with not only the latest hardware versions in its desktop and mobile computing line, but also that it performs just as admirably on older hardware with little to any loss of functionality between the two. This is a list of the Mac hardware requirements by model that officially supports Apple's latest operating system.

MacBook: 2015 and later

MacBook Pro: 2012 and later

MacBook Air: 2012 and later

Mac Mini: 2012 and later

iMac: 2012 and later

iMac Pro: 2017 and later

Mac Pro: 2013 and later, with a Metal-capable GPU

Try installing it in Safe Mode...

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

You can download it by following the link to App Store from here: https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12

Hi again,

Can you elucidate which Mac you are having the problem with, MacBook Pro or Mac Pro?

Yes meaning you tried that, or yes it worked, or???