A comment from Microsoft Community
Try running the built-in Defender which gives adequate protection, best Windows performance, least issues, and is from Microsoft who knows how to protect their OS best.
To get this working I had to use the following
Windows Defender not working in Windows 11
Click your Start Button, then just type PowerShell and on the resulting list, right click PowerShell and choose 'Run as Administrator'
Paste this command into PowerShell and press Enter:
Get-AppxPackage Microsoft.SecHealthUI -AllUsers | Reset-AppxPackage
When that completes, close PowerShell and restart (not shut down) your PC.
Now works fine