sudo apt-get update command is not working

The reason you're hitting those errors are related to your version of Ubuntu has reached it End of Life (EOL) on July 19, 2018. When Ubuntu passes a release to EOL, they no longer provide package updates through their mirrors. Linode mirrors directly from upstream, which is why trying to run an update through them will result in the error, too.

As explained in this Community post comment, regarding the same issue relate to trying to update Ubuntu Zesty, you can edit your /etc/apt/sources.list file to point to Ubuntu's unsupported, EOL release server:

edit /etc/apt/sources.list and change

"http://mirrors.linode.com/ubuntu" and "http://security.ubuntu.com/ubuntu"

to

"http://old-releases.ubuntu.com/ubuntu"

I'd also recommend updating your Ubuntu to a newer release, which would receive updates, like for security vulnerabilities & other bugs. I recently wrote up steps for a different ways to do that on this Community post:

How do I upgrade my existing Ubuntu installation?

I'm getting these errors on an Ubuntu 19.04, is that reached EOL?

Also when trying 'do-release-upgrade' I receive: 'Please install all available updates for your release before upgrading.'

But can't update since the repos are missing:

root@localhost:~# apt update
Ign:1 http://mirrors.linode.com/ubuntu disco InRelease
Ign:2 http://mirrors.linode.com/ubuntu disco-updates InRelease
Ign:3 http://mirrors.linode.com/ubuntu disco-backports InRelease
Err:4 http://mirrors.linode.com/ubuntu disco Release
404 Not Found [IP: 2600:3c03:1::45a4:df28 80]
Err:5 http://mirrors.linode.com/ubuntu disco-updates Release
404 Not Found [IP: 2600:3c03:1::45a4:df28 80]
Err:6 http://mirrors.linode.com/ubuntu disco-backports Release
404 Not Found [IP: 2600:3c03:1::45a4:df28 80]
Get:7 https://dl.yarnpkg.com/debian stable InRelease [17.1 kB]
Hit:8 https://deb.nodesource.com/node_10.x disco InRelease
Hit:9 http://security.ubuntu.com/ubuntu bionic-security InRelease
Err:7 https://dl.yarnpkg.com/debian stable InRelease
The following signatures were invalid: EXPKEYSIG 23E7166788B63E1E Yarn Packaging yarn@dan.cx
Hit:10 http://ppa.launchpad.net/certbot/certbot/ubuntu disco InRelease
Ign:11 http://security.ubuntu.com/ubuntu disco-security InRelease
Err:12 http://security.ubuntu.com/ubuntu disco-security Release
404 Not Found [IP: 2001:67c:1360:8001::24 80]
Hit:13 http://ppa.launchpad.net/chris-lea/redis-server/ubuntu disco InRelease
Reading package lists… Done
E: The repository 'http://mirrors.linode.com/ubuntu disco 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.
E: The repository 'http://mirrors.linode.com/ubuntu disco-updates 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.
E: The repository 'http://mirrors.linode.com/ubuntu disco-backports 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.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://dl.yarnpkg.com/debian stable InRelease: The following signatures were invalid: EXPKEYSIG 23E7166788B63E1E Yarn Packaging yarn@dan.cx
E: The repository 'http://security.ubuntu.com/ubuntu disco-security 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.

Whenever I tried to apt update, upgrade, etc I kept getting the 404 Not Found [IP: 2001:67c:1560:8008::15 80] message as above. My current Linode version is Debian (9) Stretch.

The IPv6 address for mirrors.linode.com (resolves to 2001:67c:1560:8008::15) which does not seem to work properly/at all with apt. I tested the address in my browser and IPv4 does work.

Note: I am NOT a Linux Expert and the below may NOT be good advice but it worked for me! If you break your system I don't accept any responsibility!

The way I fixed this on my system was by using the ForceIPv4 option on apt.
eg
sudo apt-get -o Acquire::ForceIPv4=true update

To make ForceIPv4 persistent:

• create the file /etc/apt/apt.conf.d/99force-ipv4 eg
  sudo nano /etc/apt/apt.conf.d/99force-ipv4
• Add Line
  Acquire::ForceIPv4 "true";

But I am using Ubuntu 22.04 and still getting

E: Failed to fetch http://mirrors.linode.com/ubuntu/pool/main/s/systemd/libnss-systemd_249.11-0ubuntu3.9_amd64.deb Could not connect to mirrors.linode.com:80 (2400:8904:1::ac69:2c13), connection timed out Could not connect to mirrors.linode.com:80 (172.105.44.19), connection timed out