出现 404 Not Found [IP: 或者出现 Failed to fetch http://…..
一般是由于直接使用国外的源无法连接的原因导致的
建议换源试一试。
执行备份命令：sudo cp /etc/apt/sources.list /etc/apt/sources.list.old
打开文件：sudo vim /etc/apt/sources.list
换源：复制源覆盖文件原来的内容（根据自己的网去尝试，选择最快的一个）
更新源：sudo apt update 如果出现依赖问题-->sudo apt -f install 更新软件：sudo apt upgrade
阿里云开源镜像站： http://mirrors.aliyun.com/]
[清华大学：http://mirrors.tuna.tsinghua.edu.cn/ (IPv4+IPv6)]
[中科大：http://mirrors.ustc.edu.cn/ (IPv4+IPv6)]
#添加阿里源
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
#添加清华源
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse multiverse
参考：https://www.cnblogs.com/Horizon-asd/p/12595723.html