|Please note Sophos is not yet supported in macOS Big Sur and the below steps might not work in all circumstances. These steps do not work on M1 / Apple Silicon Macs and will need to wait for an updated installer from Sophos.|
You may also need to grant full disk access.
Sophos Anti-virus requires approval of a System Extension by the end user before it's fully functional. On macOS 11 Big Sur, Sophos On-Access scanning will be disabled until you approve the Sophos System Extension.
After Sophos Anti-Virus installs you will see a prompt to open Security & Privacy in the System Preferences.
Go to the Apple Menu and select System Preferences->Security & Privacy
Click the lock icon in the bottom left if it is not already unlocked. You will be prompted to enter the password for an administrator account. Once unlocked, click the Allow button to allow software from Sophos. If the allow button doesn't show, follow the steps on approving kernel extensions.
You will be prompted to restart the computer. Please do so to enable the Sophos System Extension. After restarting the computer, you can open the Sophos preferences and see that On-Access scanning is now enabled.