As the error message on your Apple TV states… newer versions of the Plex clients need a newer version of PMS than the one you’re currently running (PMS 1.18.3 or newer).
From the related release announcement for Plex for Apple TV 7.11 from December 6, 2020:
NEW:
- This release requires Plex Media Server 1.18.3 or newer.
It looks like the update check on your server is giving you a bad “up to date” feedback.
PMS 1.18.2 has been released in November of 2019… the latest public release is PMS 1.21.1 from January 6, 2021.
On what platform/OS are you running this PMS of yours?
