This is quite a common issue that we have encountered in Windows 11. It seems some Anti-Virus break Microsoft Defender.
If you get the error “You’ll need a new app to open this windowsdefender link” when you attempt to open the Windows Security icon on the taskbar.
Restart your computer.
Close all applications.
Click start and start typing “powershell”
When “Windows Powershell appears”, right click and RUN AS ADMINISTRATOR
When the powershell command prompt opens, type this:
Get-AppxPackage Microsoft.SecHealthUI -AllUsers | Reset-AppxPackage
When it has completed, exit. Then restart Windows.
It should fix this issue after the restart.
More detailed instructions / alternatives:
On windows 11 do this
IGNORE THE ERRORS
On Windows 10 do this:
if it doesn’t work type this in the PowerShell prompt: