There are a couple things one can do to repair or update/install missing or broken packages. Updating your distro can sometimes break things. Something to keep in mind is that these commands should work on older versions of Ubuntu and Debian, but may differ slightly in syntax. Ex: apt vs apt-get for older Ubuntu versions.

The first command I would run would be:

sudo apt update --fix-missing

This will look for any missing packages and show you any newer versions needed.

Second:

sudo apt install -f

This will find any missing packages and fix any broken ones.

If none of these work it might be that you need to look to dkpg for troubleshooting.

The first command to run would be:

sudo dpkg --configure -a

This should fix any broken or partial packages.

Second:

sudo dpkg -l | grep ^..r

This will find any packages that dkpg has marked for reinstall or ones that are problematic.

Third:

sudo dpkg --remove --force-remove--reinstreq

With this you can remove any of the problem packages displayed when using the second command.

Finally:

sudo apt clean
sudo apt update

Clean up and you should have removed any packages giving you trouble.

What happens if you get this message?

sudo dpkg --remove --force-remove--reinstreq
dpkg: error: unknown force/refuse option 'remove--reinstreq'

@bradrice - Try sudo dpkg --remove --force-remove-reinstreq instead. I think the original command has an extra hyphen.

When trying to do the update we're getting the following errors:

Please install all available updates for your release before upgrading.

sudo apt update --fix-missing

Ign:1 http://security.ubuntu.com/ubuntu disco-security InRelease
Hit:2 http://ppa.launchpad.net/certbot/certbot/ubuntu disco InRelease
Ign:3 http://mirrors.linode.com/ubuntu disco InRelease
Ign:4 http://mirrors.linode.com/ubuntu disco-updates InRelease
Ign:5 http://mirrors.linode.com/ubuntu disco-backports InRelease
Err:6 http://mirrors.linode.com/ubuntu disco Release
404 Not Found [IP: 2a01:7e00:1::b24f:af37 80]
Err:7 http://security.ubuntu.com/ubuntu disco-security Release
404 Not Found [IP: 2001:67c:1360:8001::23 80]
Err:8 http://mirrors.linode.com/ubuntu disco-updates Release
404 Not Found [IP: 2a01:7e00:1::b24f:af37 80]
Err:9 http://mirrors.linode.com/ubuntu disco-backports Release
404 Not Found [IP: 2a01:7e00:1::b24f:af37 80]
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://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.
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.

Looks like the servers to upgrade from are no longer in operation.

Any ideas on this?

Thanks.

It looks like the issue you're running into is you're trying to run updates to a release that's reached its End Of Life (EOL). When a release reaches EOL, updates are available, though it's strongly recommended to upgrade to a release that is still being supported.

To get updates on an EOL'd release, you'll need to change the source you're pulling updates from in sources.list. Ubunutu has this process documented on their website:

Ubuntu EOL Upgrades

Once you update your sources.list setting, you'll be pulling from Ubuntu's mirrors, not the Linode mirrors. You may see slower speeds as a result.