将官方源换成清华源后，下载某些包出错

sudo apt update
Reading package lists... Done                      
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn focal 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 'https://cloud.r-project.org/bin/linux/ubuntu focal-cran40/ Release' no longer has 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.

解决方案

换成阿里云源

https://developer.aliyun.com/mirror/ubuntu

还源方法：
在terminal中输入gedit /etc/apt/sources.list

ubuntu20.04（focal）

deb https://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse

# deb https://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src https://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse

ubuntu 18.04(bionic) 配置如下

deb https://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse

# deb https://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
# deb-src https://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse