With the release of macOS 10.15 Apple has blocked the install onto the MacPro 5,1. With all the updated hardware on my MP51 (Metal Capable GPU, and newer WiFi/BT card), there is no valid reason for this limitation. Thankfully the members of the Mac Rumors forums (specifically dosdude1) have created a “patcher” tool that allows for installing Catalina on nearly all of the recently “Apple-Blocked” models.
My MP51 has been happily running Catalina for weeks now. Kudos to dosdude1.