Dear @Jack Kaufman
On Windows Server 2019 and later with Desktop Experience, Microsoft Defender Antivirus is included with the operating system. The Windows Security app is not installed as a separate feature that you need to add manually.
First, I would check whether Microsoft Defender Antivirus is actually installed and running. Open PowerShell as Administrator and run:
Get-MpComputerStatus | Select-Object AMRunningMode, AntivirusEnabled, AntivirusSignatureLastUpdated
If AntivirusEnabled is True, Defender Antivirus is installed and running.
If Defender has actually been removed, Microsoft provides the following command to reinstall it:
Install-WindowsFeature -Name Windows-Defender
After that, restart the server and check the status again.
You can also verify whether the Defender feature is installed with:
Get-WindowsFeature -Name Windows-Defender
Microsoft documentation: https://learn.microsoft.com/en-us/defender-endpoint/microsoft-defender-antivirus-windows-server-configure
Best regards,
Bence