DEB-based repository is not up to date
Plex download page shows as latest public version 126.96.36.19942-3ba6662e9 from June 6, 2018
If I run
sudo apt-get update && apt-cache madison plexmediaserver it shows as latest version
plexmediaserver | 188.8.131.5223-31d3c0c65 | https://downloads.plex.tv/repo/deb public/main amd64 Packages
The server is configured as shown in the support article.
I saw in an old question this has happened before, and required a large hammer to fix...?