This might happen if you used software like CCleaner to uninstall Plex server previously.
Another possibility is that the “Plex updater service” is not running.
Did you move the Plex data folder away from its default location? If you did, you probably forgot to move the registry branch for this service as well. See the “addendum” part of this post: [HowTo] An extended guide on how to move the Plex data folder on Windows
