If macOS Big Sur 11.0.1 is installed into the same APFS container as previous versions of macOS Catalina 10.15, system software updates can no longer be installed on the previous versions of macOS. (64411484) Workaround: Update the previous version of macOS Catalina to 10.15.6 or later.
Running into an issue installing Big Sur on top of Catalina 10.15.7 on 2019 Mac Pro. Downloads the installer no problem, but after a bit running – runs into a very generic dialog with Code Block An error occured installing macOS. You can try again to complete the installation.