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kingcanty@kingcanty-HP-Pavilion-17-Notebook-PC:~$ sudo apt-get update

Hit:1 ht tp://mirror.team-cymru. org/ubuntu cosmic InRelease

Ign:2 ht tp://security.debian. org cosmic InRelease                                                                                                                            

Err:3 ht tp://security.debian. org cosmic Release                                                                                                                              

  404  Not Found [IP: 151.101.184.204 80]

Hit:4 ht tp://mirror.team-cymru. org/ubuntu cosmic-updates InRelease                                                                                     

Hit:5 ht tp://mirror.team-cymru. org/ubuntu cosmic-backports InRelease                                                                                   

Hit:6 ht tp://mirror.team-cymru. org/ubuntu cosmic-security InRelease                                                              

Hit:7 ht tp://ppa.launchpad.net/danielrichter2007/grub-customizer/ubuntu cosmic InRelease                                         

Get:8 ht tp://security.ubuntu.com/ubuntu cosmic-security InRelease [88.7 kB]                           

Reading package lists... Done                                 

E: The repository 'ht tp://security.debian.org cosmic 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.

kingcanty@kingcanty-HP-Pavilion-17-Notebook-PC:~$ 

kubuntu 18.10

# deb cdrom:[Kubuntu 18.10 _Cosmic Cuttlefish_ - Release amd64 (20181017.2)]/ cosmic main multiverse restricted universe

# deb cdrom:[Kubuntu 18.10 _Cosmic Cuttlefish_ - Release amd64 (20181017.2)]/ cosmic main multiverse restricted universe

deb ht tp://mirror.team-cymru.org/ubuntu/ cosmic main restricted universe

deb ht tp://security.ubuntu.com/ubuntu/ cosmic-security main restricted universe

deb ht tp://mirror.team-cymru.org/ubuntu/ cosmic-updates main restricted universe



deb ht tp://security.ubuntu.com/ubuntu cosmic-security main restricted



# See ht tp://help.ubuntu.com/community/UpgradeNotes for how to upgrade to

# newer versions of the distribution.

#deb ht tp://mirror.team-cymru.org/ubuntu/ cosmic restricted

# deb-src ht tp://us.archive.ubuntu.com/ubuntu/ cosmic main restricted



## Major bug fix updates produced after the final release of the

## distribution.

#deb ht tp://mirror.team-cymru.org/ubuntu/ cosmic-updates restricted

# deb-src ht tp://us.archive.ubuntu.com/ubuntu/ cosmic-updates main restricted



## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu

## team. Also, please note that software in universe WILL NOT receive any

## review or updates from the Ubuntu security team.

# deb-src ht tp://us.archive.ubuntu.com/ubuntu/ cosmic universe

# deb-src ht tp://us.archive.ubuntu.com/ubuntu/ cosmic-updates universe



## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 

## team, and may not be under a free licence. Please satisfy yourself as to 

## your rights to use the software. Also, please note that software in 

## multiverse WILL NOT receive any review or updates from the Ubuntu

## security team.

deb ht tp://mirror.team-cymru.org/ubuntu/ cosmic multiverse

# deb-src ht tp://us.archive.ubuntu.com/ubuntu/ cosmic multiverse

deb ht tp://mirror.team-cymru.org/ubuntu/ cosmic-updates multiverse

# deb-src ht tp://us.archive.ubuntu.com/ubuntu/ cosmic-updates multiverse



## N.B. software from this repository may not have been tested as

## extensively as that contained in the main release, although it includes

## newer versions of some applications which may provide useful features.

## Also, please note that software in backports WILL NOT receive any review

## or updates from the Ubuntu security team.

deb ht tp://mirror.team-cymru.org/ubuntu/ cosmic-backports main restricted universe multiverse

# deb-src ht tp://us.archive.ubuntu.com/ubuntu/ cosmic-backports main restricted universe multiverse



## Uncomment the following two lines to add software from Canonical's

## 'partner' repository.

## This software is not part of Ubuntu, but is offered by Canonical and the

## respective vendors as a service to Ubuntu users.

# deb ht tp://archive.canonical.com/ubuntu cosmic partner

# deb-src ht tp://archive.canonical.com/ubuntu cosmic partner



# deb ht tp://security.ubuntu.com/ubuntu cosmic-security main restricted

# deb-src ht tp://security.ubuntu.com/ubuntu cosmic-security main restricted

# deb ht tp://security.ubuntu.com/ubuntu cosmic-security universe

# deb-src ht tp://security.ubuntu.com/ubuntu cosmic-security universe

deb ht tp://mirror.team-cymru.org/ubuntu/ cosmic-security multiverse

# deb-src ht tp://security.ubuntu.com/ubuntu cosmic-security multiverse

the 7th line was the problem, it was changed from
'deb ht tp://security.debian.org/ cosmic universe restricted main'

result
kingcanty@kingcanty-HP-Pavilion-17-Notebook-PC:~$ sudo apt-get update
[sudo] password for kingcanty:
Hit:1 http://mirror.team-cymru.org/ubuntu cosmic InRelease
Hit:2 http://mirror.team-cymru.org/ubuntu cosmic-updates InRelease

Hit:3 http://mirror.team-cymru.org/ubuntu cosmic-backports InRelease

Hit:4 http://mirror.team-cymru.org/ubuntu cosmic-security InRelease

Hit:5 http://ppa.launchpad.net/danielrichter2007/grub-customizer/ubuntu cosmic InRelease

Get:6 http://security.ubuntu.com/ubuntu cosmic-security InRelease [88.7 kB]

Fetched 88.7 kB in 2s (53.3 kB/s)

Reading package lists... Done
kingcanty@kingcanty-HP-Pavilion-17-Notebook-PC:~$

It looks like your APT sources.list contains a reference to security.debian.org, which is for Debian, not Ubuntu. While their repository is compatible with APT, you won't find anything for your operating system there. In particular, the error message indicates that they have no release called "cosmic".

The line that causes the problem appears to be:

http://security.debian.org cosmic Release