当我使用清华源的时候出现了下面的错误

Get:1 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye InRelease [75.1 kB]
Get:2 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye-updates InRelease [44.0 kB]
Ign:3 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye-backports InRelease
Get:4 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security InRelease [27.2 kB]
Get:5 https://mirrors.tuna.tsinghua.edu.cn/proxmox/debian bullseye InRelease [2,768 B]
Err:6 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye-backports Release
  404  Not Found [IP: 101.6.15.130 443]
Get:7 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/non-free Sources [81.0 kB]
Get:8 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/main Sources [8,500 kB]
Get:9 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/contrib Sources [43.2 kB]
Get:10 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/main amd64 Packages [8,066 kB]
Get:11 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/main Translation-en [6,235 kB]
Get:12 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/contrib amd64 Packages [50.4 kB]
Get:13 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/contrib Translation-en [46.9 kB]
Get:14 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/non-free amd64 Packages [96.4 kB]
Get:15 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye/non-free Translation-en [92.5 kB]
Get:16 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye-updates/main Sources [7,908 B]
Get:17 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye-updates/main amd64 Packages [18.8 kB]
Get:18 https://mirrors.tuna.tsinghua.edu.cn/debian bullseye-updates/main Translation-en [10.5 kB]
Get:19 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/contrib Sources [1,128 B]
Get:20 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/non-free Sources [1,352 B]
Get:21 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/main Sources [251 kB]
Get:22 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/main amd64 Packages [391 kB]
Get:23 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/main Translation-en [260 kB]
Get:24 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/contrib amd64 Packages [2,880 B]
Get:25 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/contrib Translation-en [2,512 B]
Get:26 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/non-free amd64 Packages [1,184 B]
Get:27 https://mirrors.tuna.tsinghua.edu.cn/debian-security bullseye-security/non-free Translation-en [1,208 B]
Get:28 https://mirrors.tuna.tsinghua.edu.cn/proxmox/debian bullseye/pve-no-subscription amd64 Packages [452 kB]
Reading package lists... Done
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/debian bullseye-backports 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.

然后我又换成了中科大的源,结果问题依旧

Hit:1 https://mirrors.ustc.edu.cn/debian bullseye InRelease
Hit:2 https://mirrors.ustc.edu.cn/debian bullseye-updates InRelease
Ign:3 https://mirrors.ustc.edu.cn/debian bullseye-backports InRelease
Get:4 https://mirrors.ustc.edu.cn/debian-security bullseye-security InRelease [27.2 kB]
Hit:5 https://mirrors.ustc.edu.cn/proxmox/debian/pve bullseye InRelease
Err:6 https://mirrors.ustc.edu.cn/debian bullseye-backports Release
  404  Not Found [IP: 218.104.71.170 443]
Reading package lists... Done
E: The repository 'https://mirrors.ustc.edu.cn/debian bullseye-backports 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.

后来网上搜了一下才知道是因为 bookworm-backports 为 Debian 12（bookworm）提供新的软件包，而bullseye-backports 已达到使用寿命，不再支持或更新
详细内容可以查看 => https://backports.debian.org/Instructions/

解决方案

使用归档的源信息即可
如果使用的是中科大的源，将/etc/apt/source.list中的 deb https://mirrors.ustc.edu.cn/debian/ bullseye-backports main contrib non-free
替换为 deb https://archive.debian.org/debian bullseye-backports main后再执行apt update即可