当我在sudo apt update安装了 MAAS 的 Ubunutu 16.04 LTS 服务器上运行时，收到以下错误消息：
graeme@MAAS:~$ sudo apt update
Hit:1 http://gb.archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://security.ubuntu.com/ubuntu xenial-security InRelease
Ign:3 http://ppa.launchpad.net/maas/stable/ubuntu xenial InRelease
Hit:4 http://gb.archive.ubuntu.com/ubuntu xenial-updates InRelease
Err:5 http://ppa.launchpad.net/maas/stable/ubuntu xenial Release
404 Not Found
Hit:6 http://gb.archive.ubuntu.com/ubuntu xenial-backports InRelease
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/maas/stable/ubuntu xenial 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 …