On a Ubuntu 21.04 the sudo apt update command generates console ouput as follows:
barium@berta$ sudo apt update
...
Ign:3 http://security.debian.org stable/updates InRelease
Err:4 http://security.debian.org stable/updates Release
404 Not Found [IP: 2a04:4e42:6c::644 80]
...
Hit:9 http://archive.ubuntu.com/ubuntu hirsute-security InRelease
Reading package lists... Done
E: The repository 'http://security.debian.org stable/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.
Manual changes to the Ubuntu Software & Updates Manager produce the same error message.
The /etc/apt/sources.list contains among others the following deb statements:
barium@berta$ cat /etc/apt/sources.list
...
deb http://archive.ubuntu.com/ubuntu hirsute-security main restricted
deb http://archive.ubuntu.com/ubuntu hirsute-security universe
deb http://archive.ubuntu.com/ubuntu hirsute-security multiverse
...
In an answer to a similar post it was suggested to replace 'archive.ubuntu.com' and 'security.ubuntu.com' with 'old-releases.ubuntu.com'. That didn't work.
