Ошибка: нет выпуска xenial для kernel-ppa [дубликат]

Полная ошибка:

Err:11 http://ppa.launchpad.net/kernel-ppa/ppa/ubuntu xenial Release
  404  Not Found
Reading package lists... Done                      
E: The repository 'http://ppa.launchpad.net/kernel-ppa/ppa/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 creation and user configuration details.

Нормальный sources.list :

$ cat /etc/apt/sources.list
deb http://ports.ubuntu.com/ xenial main restricted universe multiverse
deb-src http://ports.ubuntu.com/ xenial main restricted universe multiverse

deb http://ports.ubuntu.com/ xenial-updates main restricted universe multiverse
deb-src http://ports.ubuntu.com/ xenial-updates main restricted universe multiverse

deb http://ports.ubuntu.com/ xenial-security main restricted universe multiverse
deb-src http://ports.ubuntu.com/ xenial-security main restricted universe multiverse

deb http://ports.ubuntu.com/ xenial-backports main restricted universe multiverse
deb-src http://ports.ubuntu.com/ xenial-backports main restricted universe multiverse

Я вижу, что в kernel-ppa нет папки xenial dist . Как я могу исправить эту ошибку или отключить выдачу ошибки из-за отсутствия папки xenial?