原因是由于Debian官方将Debian软件源由默认站点deb.debian.org移至存档站点archive.debian.org，基于官方软件源的镜像站同步了这一修改， 导致Debian镜像在运行apt update命令时会报错： buster Release 404 Not Found 。 

解决方法是：切换至Debian归档仓库或者更新至最新系统

1.保留当前版本环境，不升级系统，修改/etc/apt/sources.list 将其中域名deb.debian.org 更改为归档源archive.debian.org，其他可不做更改：

deb https://archive.debian.org/debian buster main contrib non-free
deb-src https://archive.debian.org/debian buster main contrib non-free
deb https://archive.debian.org/debian-security buster/updates main contrib non-free
deb-src https://archive.debian.org/debian-security buster/updates main contrib non-free
deb https://archive.debian.org/debian buster-updates main contrib non-free
deb-src https://archive.debian.org/debian buster-updates main contrib non-free

2.升级至最新系统命令 apt upgrade