Update Issue No Public Key for Plex Repo

[loopback99]

Install Updates Error log
===========================
Install Updates could not fetch the package cache information lists.
Go to https://www.freecinema2022.gq/forums/ and paste the log below into a new or existing thread for assistance.

============ Log ===========

Hit:1 http://repo.linuxliteos.com/linuxlite emerald InRelease
Get:2 https://downloads.plex.tv/repo/deb public InRelease [6,685 B]
Hit:3 http://us.archive.ubuntu.com/ubuntu focal InRelease
Hit:4 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:5 http://us.archive.ubuntu.com/ubuntu focal-updates InRelease
Err:2 https://downloads.plex.tv/repo/deb public InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 97203C7B3ADCA79D
Hit:6 http://ppa.launchpad.net/gambas-team/gambas3/ubuntu focal InRelease
Hit:7 http://us.archive.ubuntu.com/ubuntu focal-backports InRelease
Hit:8 http://archive.canonical.com/ubuntu focal InRelease
Reading package lists...
W: GPG error: https://downloads.plex.tv/repo/deb public InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 97203C7B3ADCA79D
E: The repository 'https://downloads.plex.tv/repo/deb public InRelease' is not signed.