Here is the terminal output when I enter
$ sudo apt update
...
Ign:1 https://mirrors.sonic.net/pureos/repo/pureos byzantium InRelease
Ign:2 https://mirrors.sonic.net/pureos/repo/pureos byzantium-security InRelease
Ign:3 https://mirrors.sonic.net/pureos/repo/pureos byzantium-updates InRelease
Err:4 https://mirrors.sonic.net/pureos/repo/pureos byzantium Release
Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 157.131.0.16 443]
Err:5 https://mirrors.sonic.net/pureos/repo/pureos byzantium-security Release
Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 157.131.0.16 443]
Err:6 https://mirrors.sonic.net/pureos/repo/pureos byzantium-updates Release
Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 157.131.0.16 443]
Reading package lists... Done
E: The repository 'https://mirrors.sonic.net/pureos/repo/pureos byzantium 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://mirrors.sonic.net/pureos/repo/pureos byzantium-security 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://mirrors.sonic.net/pureos/repo/pureos byzantium-updates 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.