Hi,
sometimes it happens that the Windows Security App won’t open.
The possible solutions
Reset the Appx package
PS D:\> Get-AppxPackage Microsoft.Windows.SecHealthUI | Reset-AppxPackage
Or reinstall the Appx package for the logged on user
PS D:\> Add-AppxPackage -DisableDevelopmentMode -Register ((Get-AppxPackage Microsoft.Windows.SecHealthUI).InstallLocation + '\AppxManifest.xml')
Michael