Update Error Message - NO_PUBKEY

[Szon28]

Many thanks for your reply.

I have uninstalled Handbrake completely in the Package Manager but I still get the same updates error message:

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://gb.archive.ubuntu.com/ubuntu focal InRelease
Hit:2 http://gb.archive.ubuntu.com/ubuntu focal-security InRelease
Hit:3 http://archive.canonical.com/ubuntu focal InRelease
Get:4 http://ppa.launchpad.net/stebbins/handbr...ses/ubuntu focal InRelease [17.5 kB]
Hit:5 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:6 http://repo.linuxliteos.com/linuxlite emerald InRelease
Err:4 http://ppa.launchpad.net/stebbins/handbr...ses/ubuntu focal InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8771ADB0816950D8
Reading package lists...
W: GPG error: http://ppa.launchpad.net/stebbins/handbr...ses/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8771ADB0816950D8
E: The repository 'http://ppa.launchpad.net/stebbins/handbrake-releases/ubuntu focal InRelease' is not signed.

What else can I do? I am not an expert at all. What is ppa.launchpad.net/stebbins/handbrake ?? Do I need it?

Many thanks for helping.