sudo apt updateUbuntu 20.04 中的输出：
Ign:1 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:2 http://np.archive.ubuntu.com/ubuntu focal InRelease
Hit:3 http://np.archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:4 http://np.archive.ubuntu.com/ubuntu focal-backports InRelease
Err:5 http://security.ubuntu.com/ubuntu focal-security Release 404 Not Found [IP: 110.44.112.44 80]
Hit:6 http://ppa.launchpad.net/deadsnakes/ppa/ubuntu focal InRelease
Hit:7 http://ppa.launchpad.net/micahflee/ppa/ubuntu focal InRelease
Reading package lists... Done
E: The repository 'http://security.ubuntu.com/ubuntu focal-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.
/etc/apt/sources.list和软件和更新没有这个 ppa。