Apple made some significant changes under the bonnet of the macOS operating system called Big Sur. As of Big Sur's release in November 2020, the antivirus programme we use - Sophos Central - is not compatible and will not protect a computer from viruses. Additionally the VPN client we use doesn't work properly.
Until Sophos and Fortinet produce versions that support it CSCS will be blocking the installation of Big Sur. This is likely to last until the end of March 2021 but will be regularly reviewed.
If you try to install Big Sur on a computer with Sophos Central you will be able to download it, but when you try to run the installer you will see a message advising that it has been blocked.
If you click details you'll see more details saying that "use of application macOS Big Sur Installer has been blocked by your administrator"
If you absolutely must install Big Sur, uninstall Sophos Central. Install the University's MacAfee AntiVirus (which is free for Cambridge staff and students) https://help.uis.cam.ac.uk/service/security/antivirus/mac. Then do the Big Sur update.