I always hate to get to the bottom of a forum thread only to find it unsolved.
I had a similar issue and found that the proxmox-release-bullseye.gpg file that was created in the previous step needed additional permissions.
root@debian:/etc/apt/trusted.gpg.d# ls -lah
total 80K
drwxr-xr-x 2 root root 4.0K Feb 11 00:36 .
drwxr-xr-x 7 root root 4.0K Sep 30 10:07 ..
-rw-r--r-- 1 root root 8.5K Mar 16 2021 debian-archive-bullseye-automatic.gpg
-rw-r--r-- 1 root root 8.6K Mar 16 2021 debian-archive-bullseye-security-automatic.gpg
-rw-r--r-- 1 root root 2.4K Mar 16 2021 debian-archive-bullseye-stable.gpg
-rw-r--r-- 1 root root 8.0K Mar 16 2021 debian-archive-buster-automatic.gpg
-rw-r--r-- 1 root root 8.0K Mar 16 2021 debian-archive-buster-security-automatic.gpg
-rw-r--r-- 1 root root 2.3K Mar 16 2021 debian-archive-buster-stable.gpg
-rw-r--r-- 1 root root 7.3K Mar 16 2021 debian-archive-stretch-automatic.gpg
-rw-r--r-- 1 root root 7.3K Mar 16 2021 debian-archive-stretch-security-automatic.gpg
-rw-r--r-- 1 root root 2.3K Mar 16 2021 debian-archive-stretch-stable.gpg
-rw------- 1 root root 1.2K Dec 14 2020 proxmox-release-bullseye.gpg
After changing them with "chmod go+r /etc/apt/trusted.gpg.d/proxmox-release-bullseye.gpg"
root@debian:/etc/apt/trusted.gpg.d# ls -lah
total 80K
drwxr-xr-x 2 root root 4.0K Feb 11 00:36 .
drwxr-xr-x 7 root root 4.0K Sep 30 10:07 ..
-rw-r--r-- 1 root root 8.5K Mar 16 2021 debian-archive-bullseye-automatic.gpg
-rw-r--r-- 1 root root 8.6K Mar 16 2021 debian-archive-bullseye-security-automatic.gpg
-rw-r--r-- 1 root root 2.4K Mar 16 2021 debian-archive-bullseye-stable.gpg
-rw-r--r-- 1 root root 8.0K Mar 16 2021 debian-archive-buster-automatic.gpg
-rw-r--r-- 1 root root 8.0K Mar 16 2021 debian-archive-buster-security-automatic.gpg
-rw-r--r-- 1 root root 2.3K Mar 16 2021 debian-archive-buster-stable.gpg
-rw-r--r-- 1 root root 7.3K Mar 16 2021 debian-archive-stretch-automatic.gpg
-rw-r--r-- 1 root root 7.3K Mar 16 2021 debian-archive-stretch-security-automatic.gpg
-rw-r--r-- 1 root root 2.3K Mar 16 2021 debian-archive-stretch-stable.gpg
-rw-r--r-- 1 root root 1.2K Dec 14 2020 proxmox-release-bullseye.gpg
All was right in the world again and the install proceeded as expected.
