In the Windows 11 Beta, I experienced an issue with Windows Security (fka. Windows Defender) not working.
The shield would appear in the system tray but clicking on it or opening Windows Security from the Start menu wouldn't do anything.
To fix this, open Powershell as Admin and copy paste the following:
Get-AppxPackage Microsoft.SecHealthUI -AllUsers | Reset-AppxPackage
Once you do that, you can go to: Start -> Settings >> Privacy & Security and click on "Open Windows Security" or type "Windows Security" into the Start Menu.