I have Kubuntu 18.04.01 installed on my PC. I'm a computer tech, but I have extremely limited knowledge of Linux. Everything is running fine, but I can no longer update the system.
sudo command doesn't work in the terminal and the Software Center fails to update. I am entering the correct password, but the system fails to update. The Software Center does successfully find the package to update. I get an error message from the Software Center, but it is a light grey background with dark grey text. I'm unable to read it before it shuts off.
Below is the text from the terminal after the root login fails.
sudo apt-get update
[sudo] password for glen:
Hit:1 repo.steampowered.com/steam precise InRelease
Hit:2 us.archive.ubuntu.com/ubuntu bionic InRelease
Get:3 security.ubuntu.com/ubuntu bionic-security InRelease [83.2 kB]
Get:4 downloads.plex.tv/repo/deb public InRelease [3,408 B]
Ign:5 dl.google.com/linux/chrome/deb stable InRelease
Hit:6 us.archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:7 us.archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:8 dl.google.com/linux/chrome/deb stable Release
Err:4 downloads.plex.tv/repo/deb public InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 97203C7B3ADCA79D
Reading package lists... Done
W: GPG error: 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 'downloads.plex.tv/repo/deb public InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
