Did you follow a some guide when you set this up? The repo line doesn’t look default, nor does the retrieval or storage of the key.
The first step to deploying is sudo dpkg -i installing the package you can download here:
https://www.plex.tv/media-server-downloads/?cat=computer&plat=linux#plex-media-server
After deploying that, an apt source will be placed in /etc/apt/sources.list.d for PMS. If you open the plexmediaserver.list file there you only need to uncomment the source line and install the key as instructed in the comments.
I’m not suggesting that what you’re doing is necessarily wrong, but when we have issues it’s best to fall back to best practices.
Note: Updating via apt only supports stable/general release versions. You don’t have the option to install Plex Pass betas via apt.
