The latest compatibility matrix indicates the 2019 is not certified for Plex 7.21 yet. When it will be certified?
Release : 7.21
Component : CA Plex
Windows Server 2019 Has been certified for use with CA Plex 7.2.1.
https://techdocs.broadcom.com/content/dam/broadcom/techdocs/us/en/dita/ca-enterprise-software/intelligent-automation/ca-plex/plex721/topics/CAPlex-Compatibility-Matrix.pdf