I also got his error today for ' apt-get update':
Err:8 http://imola.armbian.com/apt bullseye Release
404 Not Found [IP: 93.103.15.56 80]
Reading package lists... Done
E: The repository 'http://apt.armbian.com bullseye 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.