You might run into this error on Windows Server 2019 when trying to open Windows Defender to change/validate its settings:

You'll need a new app to open this windowsdefender

When this happens just execute the following line in an elevated PowerShell console

Add-AppxPackage -Register -DisableDevelopmentMode "C:\Windows\SystemApps\Microsoft.Windows.SecHealthUI_cw5n1h2txyewy\AppXManifest.xml"

And the results (I need this on two of my server in my Lab):

Windows Defender working on Windows Server 2019