Apt 说所有软件包都没有任何安装候选

Question

我的电脑上安装了 Ubuntu MATE 20.04。不知何故，apt 现在完全坏了。每一个软件包没有安装候选项。

更糟糕的是，我根本无法使用 Synaptic 或 aptitude 下载任何软件包。/etc/apt/sources.list似乎完全没问题。

突触视图。

APT 错误（西班牙语）

[sudo] password for disablegraphics:  
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package fluid 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 'fluid' has no installation candidate
disablegraphics@disablegraphics:~$ update-manager
Traceback (most recent call last):
  File "/usr/bin/update-manager", line 28, in <module>
    import gi
ModuleNotFoundError: no module named 'gi'

Sources.list 第 1 部分：

Sources.list 第 2 部分：