大约 4 个月前，我升级到了 20.04。我从不断向我发送升级消息的页面下载了它，并在新的 SSD 上进行了全新安装。一切运行良好，过去几个月从南非服务器多次更新软件。我现在在北非。我尝试加载 DCP-T300 的 Brother 打印机驱动程序。它说我需要更新。更新说无法从 SA 服务器下载。我改用主服务器。它整整一个星期都给我带来问题，最后，在一篇帖子上，我看到了运行cat /etc/apt/sources.list
# deb cdrom:[Ubuntu 20.04 LTS _Focal Fossa_ - Release amd64 (20200423)]/ focal main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu focal universe
# deb-src http://za.archive.ubuntu.com/ubuntu/ focal universe
deb http://archive.ubuntu.com/ubuntu focal-updates universe
# deb-src http://za.archive.ubuntu.com/ubuntu/ focal-updates universe
This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu focal partner
# deb-src http://archive.canonical.com/ubuntu focal partner
# This system was installed using small removable media
# (e.g. netinst, live or single CD). The matching "deb cdrom"
# entries were disabled at the end of the installation process.
# For information about how to configure apt package sources,
# see the sources.list(5) manual.
Err:1 http://archive.ubuntu.com/ubuntu focal InRelease
506 Failure To Connect To Web Server [IP: 91.189.88.152 80]
Err:2 http://archive.ubuntu.com/ubuntu focal-updates InRelease
506 Failure To Connect To Web Server [IP: 91.189.88.152 80]
Err:3 http://archive.ubuntu.com/ubuntu focal-backports InRelease
506 Failure To Connect To Web Server [IP: 91.189.88.152 80]
Err:4 http://archive.ubuntu.com/ubuntu focal-security InRelease
506 Failure To Connect To Web Server [IP: 91.189.88.152 80]
Reading package lists... Done
N: See apt-secure(8) manpage for repository creation and user configuration details.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
E: The repository 'http://archive.ubuntu.com/ubuntu focal InRelease' is not signed.
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal/InRelease 506 Failure
我该如何解决？