Clean install unused ID Apple
You should try to find the model.  The only Mac Pro not capable of running the latest OS is the MacPro1,1.  I think they were from 2006-7.  

Older computers which existed before 'free' OSX upgrades had to purchase them via the App Store.  It's possibly trying to verify an Apple ID that was used for this.  

If you have access to another Mac, you can build a bootable USB with an OSX installer.  First you need to obtain the Install OS X app. As per Apple:


  Use these steps to download the version of OS X you want to install.
  From the Apple menu, choose App Store.  Hold the Option key and click
  the Purchases tab. You should see the versions of OS X you've
  previously purchased. If you don't see your purchases, log in to the
  App Store with the ID you previously used to install OS X.  Click the
  Install button next to the version of OS X you want to download. The
  Mac App Store normally displays OS X as being "Installed" in the
  purchases pane. This prevents you from downloading it when your Mac
  already has the same version of OS X installed. When you hold the
  Option key as you click the Purchases tab, the button to the right of
  the item you want to download should change to say "Install". If it
  doesn't, use Spotlight to search for "Install OS X" on your computer.
  It's likely you already have a copy of the same OS X installer in the
  Applications folder, or on a connected drive.


(I'll just quickly summarize the rest; if you get this far, and have questions, just respond and we'll try our best.)  Once you have the installer, you'll also need at least an 8 GB flash drive.  Wipe it and format as GUID and HFS.  Then follow these instructions to create a bootable installer.  Once done, plug it into the Mac, hold option while booting, and select the USB drive. The rest is self explanatory.