When I try to upgrade my Ubuntu MATE operating system from 22.04 to Ubuntu 24.04.1. I receive numerous NO_PUBKEY errors. I don't want to save anything on my 22.04 desktop version of Ubuntu except a Windows 7 KVM-QEMU virtual machine during the upgrade. My machine is a home built AMD Ryzen 7 3700x with Gigabyte x 570 Gaming X motherboard and Samsung 980 Pro 2 TB. Also, I have no idea what is a "key" or "keyring" or whatever.
Here are the errors I get when I type sudo apt update:
[sudo] password for richard:
Get:1 http://us.archive.ubuntu.com/ubuntu jammy InRelease [270 kB]
Get:2 http://security.ubuntu.com/ubuntu jammy-security InRelease [129 kB]
Err:1 http://us.archive.ubuntu.com/ubuntu jammy InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Err:2 http://security.ubuntu.com/ubuntu jammy-security InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
Reading package lists... Done
W: GPG error: http://us.archive.ubuntu.com/ubuntu jammy InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
E: The repository 'http://us.archive.ubuntu.com/ubuntu jammy 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.
W: GPG error: http://security.ubuntu.com/ubuntu jammy-security InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 871920D1991BC93C
E: The repository 'http://security.ubuntu.com/ubuntu jammy-security 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.
richard@X570-GAMING-X:~$
