If you are updating from Debian Buster to Debian Bullseye, you may notice the message in the title when editing you sources.list file. This is because the repository has changed.
In buster it was:
deb http://security.debian.org/ buster/updates main contrib non-free
deb http://deb.debian.org/debian buster/security main
In bullseye it is
deb http://deb.debian.org/debian-security/ bullseye-security main
or
deb http://security.debian.org/debian-security bullseye-security main
depending on where you look. (Debian Security Information or Debian SourcesList.)
Simply replacing Buster with Bullseye gives the error in the title.
Edit: changed the Buster repository because I think I misremembered the address. I changed to one I blogged about before as working after a similar issue.
Debian seems to be trying to make the security repository address as confusing as possible, and succeeding.
