Thank You
You appear to be running Plex Media Server as a Windows Service or as Scheduled Task. The updater is not supported currently with such a setup.
I do run Plex Media Service as a service as well and I do all my updates manually - stopping plex, updating it and ensuring i do not have auto start on login and then starting Plex - I use this product from @cjmurph - PMS As A Service
We are working with the developer at the moment to get the updater to work with the service. I would suggest you switch to using this 3rd party solution and look out for a future release note for Plex Media Server referencing working with the PMS Service
