我刚刚在我的 raspberry pi 上安装了 64 位 ubuntu 服务器（带有官方的 imager 程序），开始使用它，但找不到许多软件包。例如ruby，，python3-libotrrent等等，我得到的是E: Unable to locate package libboost-python或E: Package 'python3-libtorrent' has no installation candidate。您有什么解决方案吗，我之前使用过 raspbian，它能用，但速度较慢。谢谢回复！
编辑
我的 apt 更新输出：
Hit:1 http://ppa.launchpad.net/deluge-team/stable/ubuntu focal InRelease
Hit:2 http://ports.ubuntu.com/ubuntu-ports focal InRelease
Hit:3 http://ppa.launchpad.net/fish-shell/release-3/ubuntu focal InRelease
Get:4 http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease [107 kB]
Get:5 http://ports.ubuntu.com/ubuntu-ports focal-backports InRelease [98.3 kB]
Get:6 http://ports.ubuntu.com/ubuntu-ports focal-security InRelease [107 kB]
Fetched 312 kB in 2s (153 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
输出sudo apt install ruby：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package ruby is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'ruby' has no installation candidate
编辑2
添加以下行：
deb http://archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse
导致错误：
sudo apt update
Hit:1 http://ports.ubuntu.com/ubuntu-ports focal InRelease
Hit:2 http://ppa.launchpad.net/deluge-team/stable/ubuntu focal InRelease
Hit:3 http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease
Hit:4 http://ports.ubuntu.com/ubuntu-ports focal-backports InRelease
Get:5 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
Hit:6 http://ppa.launchpad.net/fish-shell/release-3/ubuntu focal InRelease
Hit:7 http://ports.ubuntu.com/ubuntu-ports focal-security InRelease
Get:8 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]
Get:9 http://archive.ubuntu.com/ubuntu focal-updates InRelease [107 kB]
Ign:10 http://archive.ubuntu.com/ubuntu focal/main arm64 Packages
Get:11 http://archive.ubuntu.com/ubuntu focal/main Translation-en [506 kB]
Get:12 http://archive.ubuntu.com/ubuntu focal/main arm64 c-n-f Metadata [28.6 kB]
Get:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages [1300 B]
Get:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages [1300 B]
Get:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages [1300 B]
Get:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages [1300 B]
Get:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages [1300 B]
Get:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages [1300 B]
Ign:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages
Ign:20 http://archive.ubuntu.com/ubuntu focal/universe arm64 Packages
Ign:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages
Ign:10 http://archive.ubuntu.com/ubuntu focal/main arm64 Packages
Ign:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages
Ign:20 http://archive.ubuntu.com/ubuntu focal/universe arm64 Packages
Ign:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages
Ign:10 http://archive.ubuntu.com/ubuntu focal/main arm64 Packages
Ign:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages
Ign:20 http://archive.ubuntu.com/ubuntu focal/universe arm64 Packages
Get:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages [139 kB]
Get:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages [139 kB]
Get:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages [139 kB]
Get:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages [139 kB]
Get:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages [139 kB]
Get:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages [139 kB]
Get:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages [139 kB]
Get:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages [139 kB]
Ign:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages
Ign:30 http://archive.ubuntu.com/ubuntu focal-updates/main arm64 Packages
Ign:31 http://archive.ubuntu.com/ubuntu focal-updates/restricted arm64 Packages
Ign:32 http://archive.ubuntu.com/ubuntu focal-updates/universe arm64 Packages
Ign:10 http://archive.ubuntu.com/ubuntu focal/main arm64 Packages
Ign:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages
Ign:20 http://archive.ubuntu.com/ubuntu focal/universe arm64 Packages
Ign:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages
Ign:30 http://archive.ubuntu.com/ubuntu focal-updates/main arm64 Packages
Ign:31 http://archive.ubuntu.com/ubuntu focal-updates/restricted arm64 Packages
Ign:32 http://archive.ubuntu.com/ubuntu focal-updates/universe arm64 Packages
Ign:10 http://archive.ubuntu.com/ubuntu focal/main arm64 Packages
Ign:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages
Ign:20 http://archive.ubuntu.com/ubuntu focal/universe arm64 Packages
Ign:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages
Ign:30 http://archive.ubuntu.com/ubuntu focal-updates/main arm64 Packages
Ign:31 http://archive.ubuntu.com/ubuntu focal-updates/restricted arm64 Packages
Ign:32 http://archive.ubuntu.com/ubuntu focal-updates/universe arm64 Packages
Err:10 http://archive.ubuntu.com/ubuntu focal/main arm64 Packages
404 Not Found [IP: 91.189.88.142 80]
Ign:33 http://security.ubuntu.com/ubuntu focal-security/main arm64 Packages
Get:34 http://security.ubuntu.com/ubuntu focal-security/main Translation-en [46.8 kB]
Get:35 http://security.ubuntu.com/ubuntu focal-security/main arm64 c-n-f Metadata [3224 B]
Get:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages [932 B]
Get:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages [932 B]
Get:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages [932 B]
Get:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages [932 B]
Get:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages [932 B]
Get:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages [932 B]
Ign:13 http://archive.ubuntu.com/ubuntu focal/restricted arm64 Packages
Ign:20 http://archive.ubuntu.com/ubuntu focal/universe arm64 Packages
Ign:21 http://archive.ubuntu.com/ubuntu focal/multiverse arm64 Packages
Ign:30 http://archive.ubuntu.com/ubuntu focal-updates/main arm64 Packages
Ign:31 http://archive.ubuntu.com/ubuntu focal-updates/restricted arm64 Packages
Ign:32 http://archive.ubuntu.com/ubuntu focal-updates/universe arm64 Packages
Ign:30 http://archive.ubuntu.com/ubuntu focal-updates/main arm64 Packages
Ign:31 http://archive.ubuntu.com/ubuntu focal-updates/restricted arm64 Packages
Ign:32 http://archive.ubuntu.com/ubuntu focal-updates/universe arm64 Packages
Err:30 http://archive.ubuntu.com/ubuntu focal-updates/main arm64 Packages
404 Not Found [IP: 91.189.88.142 80]
Ign:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages
Ign:43 http://security.ubuntu.com/ubuntu focal-security/universe arm64 Packages
Ign:33 http://security.ubuntu.com/ubuntu focal-security/main arm64 Packages
Ign:31 http://archive.ubuntu.com/ubuntu focal-updates/restricted arm64 Packages
Ign:32 http://archive.ubuntu.com/ubuntu focal-updates/universe arm64 Packages
Ign:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages
Ign:43 http://security.ubuntu.com/ubuntu focal-security/universe arm64 Packages
Ign:33 http://security.ubuntu.com/ubuntu focal-security/main arm64 Packages
Ign:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages
Ign:43 http://security.ubuntu.com/ubuntu focal-security/universe arm64 Packages
Ign:33 http://security.ubuntu.com/ubuntu focal-security/main arm64 Packages
Ign:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages
Ign:43 http://security.ubuntu.com/ubuntu focal-security/universe arm64 Packages
Ign:33 http://security.ubuntu.com/ubuntu focal-security/main arm64 Packages
Ign:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages
Ign:43 http://security.ubuntu.com/ubuntu focal-security/universe arm64 Packages
Err:33 http://security.ubuntu.com/ubuntu focal-security/main arm64 Packages
404 Not Found [IP: 91.189.91.39 80]
Ign:36 http://security.ubuntu.com/ubuntu focal-security/restricted arm64 Packages
Ign:43 http://security.ubuntu.com/ubuntu focal-security/universe arm64 Packages
Fetched 479 kB in 4s (109 kB/s)
Reading package lists... Done
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal/main/binary-arm64/Packages 404 Not Found [IP: 91.189.88.142 80]
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/binary-arm64/Packages 404 Not Found [IP: 91.189.88.142 80]
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/focal-security/main/binary-arm64/Packages 404 Not Found [IP: 91.189.91.39 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.