我无法在系统上安装 synaptic。我尝试了以下方法，但没有成功。
sudo apt-get update
sudo apt-get install synaptic
我得到输出：
Reading package lists... Done
Building dependency tree Reading state information... Done
Package synaptic 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 'synaptic' has no installation candidate
这是我运行 sudo apt update 时收到的错误消息：
cccc@cccc:~$ sudo apt update
[sudo] password for ccccc:
Ign:1 http://gb.archive.ubuntu.com/ubuntu artful InRelease
Hit:2 http://archive.canonical.com/ubuntu artful InRelease
Ign:3 http://gb.archive.ubuntu.com/ubuntu artful-updates InRelease
Ign:4 http://gb.archive.ubuntu.com/ubuntu artful-backports InRelease
Ign:5 http://gb.archive.ubuntu.com/ubuntu artful-security InRelease
Err:6 http://gb.archive.ubuntu.com/ubuntu artful Release
404 Not Found [IP: 91.189.88.149 80]
Err:7 http://gb.archive.ubuntu.com/ubuntu artful-updates Release
404 Not Found [IP: 91.189.88.149 80]
Err:8 http://gb.archive.ubuntu.com/ubuntu artful-backports Release
404 Not Found [IP: 91.189.88.149 80]
Err:9 http://gb.archive.ubuntu.com/ubuntu artful-security Release
404 Not Found [IP: 91.189.88.149 80]
Reading package lists... Done
E: The repository 'http://gb.archive.ubuntu.com/ubuntu artful 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.
E: The repository 'http://gb.archive.ubuntu.com/ubuntu artful-updates
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.
E: The repository 'http://gb.archive.ubuntu.com/ubuntu artful-
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.
E: The repository 'http://gb.archive.ubuntu.com/ubuntu artful-security
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.
有人知道如何安装它吗？