I found a report of this previously from 2020, but the solution posted there makes no sense in that it refers to repositories for the previous (buster) version. Any suggestions?
sudo apt update
Ign:1 https://apt.radxa.com/focal-stable focal InRelease
Hit:2 http://ports.ubuntu.com/ubuntu-ports focal InRelease
Ign:3 https://apt.radxa.com/focal-testing focal InRelease
Hit:4 http://ports.ubuntu.com/ubuntu-ports focal-security InRelease
Err:5 https://apt.radxa.com/focal-stable focal Release
Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 185.199.108.153 443]
Hit:6 http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease
Err:7 https://apt.radxa.com/focal-testing focal Release
Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 185.199.108.153 443]
Hit:8 http://ports.ubuntu.com/ubuntu-ports focal-backports InRelease
Reading package lists… Done
E: The repository ‘https://apt.radxa.com/focal-stable focal Release’ no longer has a Release file.
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.
E: The repository ‘https://apt.radxa.com/focal-testing focal Release’ does not have a Release file.
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.
Keys are as follows:
sudo apt-key list
/etc/apt/trusted.gpg
pub rsa3072 2020-11-24 [SC] [expires: 2022-11-24]
466B 1E7F 6809 6652 1384 8FCA B99C 4BEF EC47 E96E
uid [ unknown] Radxa dev@radxa.com
sub rsa3072 2020-11-24 [E] [expires: 2022-11-24]
/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-archive.gpg
pub rsa4096 2012-05-11 [SC]
790B C727 7767 219C 42C8 6F93 3B4F E6AC C0B2 1F32
uid [ unknown] Ubuntu Archive Automatic Signing Key (2012) ftpmaster@ubuntu.com
/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-cdimage.gpg
pub rsa4096 2012-05-11 [SC]
8439 38DF 228D 22F7 B374 2BC0 D94A A3F0 EFE2 1092
uid [ unknown] Ubuntu CD Image Automatic Signing Key (2012) cdimage@ubuntu.com
/etc/apt/trusted.gpg.d/ubuntu-keyring-2018-archive.gpg
pub rsa4096 2018-09-17 [SC]
F6EC B376 2474 EDA9 D21B 7022 8719 20D1 991B C93C
uid [ unknown] Ubuntu Archive Automatic Signing Key (2018) ftpmaster@ubuntu.com