问题描述：

一、问题原因：换源错误！

我的ubuntu版本和源版本不一致

# 1.查看sources.list
/etc/apt/sources.list
# 2.查看ubuntu版本
sudo lsb_release -a
# 3.
对于 Ubuntu 20.04 (focal)：
deb http://mirrors.cqu.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src http://mirrors.cqu.edu.cn/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.cqu.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src http://mirrors.cqu.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.cqu.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src http://mirrors.cqu.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb http://mirrors.cqu.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src http://mirrors.cqu.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb http://mirrors.cqu.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src http://mirrors.cqu.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse

# 4.换源，将上面源的复制到以下文件中
vim /etc/apt/sources.list
# 5.更新缓存和软件包
sudo apt update
sudo apt upgrade

二、安装apt

# 1. 到镜像站找到对应的apt版本，下载到本地
wget http://mirrors.163.com/ubuntu/pool/main/a/apt/apt_1.0.1ubuntu2.24_amd64.deb 

# 2. 安装
sudo dpkg -i apt_1.0.1ubuntu2.24_amd64.deb

# 3. 更新
sudo apt update
sudo apt upgrade

这样就能正常运行了。