Thanks for your help. My sources.list was already bullseye, though. I am still struggling to find the answer to the issue. I'll offer my sources files and selected output of apt update i'm receiving:
/etc/apt/sources.list:deb http://deb.debian.org/debian/ bullseye testing main contrib
non-free
deb http://deb.debian.org/debian/ bullseye testing-updates main
contrib non-free
deb http://deb.debian.org/debian-security bullseye
testing-security main
/etc/apt/sources.list.d/qubes-r4.list:
# Main qubes updates repository
deb [arch=amd64] https://deb.qubes-os.org/r4.0/vm bullseye main
#deb-src https://deb.qubes-os.org/r4.0/vm bullseye main
# Qubes updates candidates repository
deb [arch=amd64] https://deb.qubes-os.org/r4.0/vm bullseye-testing
main
deb-src https://deb.qubes-os.org/r4.0/vm bullseye-testing main
# Qubes security updates testing repository
deb [arch=amd64] https://deb.qubes-os.org/r4.0/vm
bullseye-securitytesting main
deb-src https://deb.qubes-os.org/r4.0/vm bullseye-securitytesting
main
Output of "sudo apt update"
Hit:1 http://deb.debian.org/debian bullseye
InRelease
Hit:3 https://deb.qubes-os.org/r4.0/vm bullseye
InRelease
Hit:2 http://ftp.halifax.rwth-aachen.de/kali kali-rolling
InRelease
Hit:4 https://deb.qubes-os.org/r4.0/vm bullseye-testing
InRelease
Ign:5 http://deb.debian.org/debian-security bullseye InRelease
Hit:6 https://deb.qubes-os.org/r4.0/vm bullseye-securitytesting
InRelease
Err:7 http://deb.debian.org/debian-security bullseye Release
404 Not Found [IP: 127.0.0.1 8082]
Reading package lists... Done
Then...
W: Skipping acquire of configured file
'testing-updates/i18n/Translation-en_US' as repository
'http://deb.debian.org/debian bullseye InRelease' doesn't have the
component 'testing-updates' (component misspelt in sources.list?)
E: The repository 'http://deb.debian.org/debian-security bullseye
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.
W: Target Packages (main/binary-amd64/Packages) is configured
multiple times in /etc/apt/sources.list:2 and
/etc/apt/sources.list:4
W: Target Packages (main/binary-all/Packages) is configured
multiple times in /etc/apt/sources.list:2 and
/etc/apt/sources.list:4
Any hints would be much appreciated.
Logan