sudo apt update not working on Ubuntu?

Question

sudo apt update

Get:1 file:/var/cuda-repo-ubuntu2004-11-6-local  InRelease
Ign:1 file:/var/cuda-repo-ubuntu2004-11-6-local  InRelease
Get:2 file:/var/cuda-repo-ubuntu2204-12-4-local  InRelease [1572 B]
Get:3 file:/var/cuda-repo-ubuntu2004-11-6-local  Release [564 B]
Get:2 file:/var/cuda-repo-ubuntu2204-12-4-local  InRelease [1572 B]
Get:3 file:/var/cuda-repo-ubuntu2004-11-6-local  Release [564 B]
Hit:5 nvidia.github.io/libnvidia-container/stable/deb/amd64  InRelease
Hit:6 developer.download.nvidia.com/compute/cuda/repos/ubuntu2004/x86_64  InRelease
Hit:7 developer.download.nvidia.com/compute/cuda/repos/ubuntu2204/x86_64  InRelease
Hit:8 download.docker.com/linux/ubuntu focal InRelease
Hit:9 artifacts.elastic.co/packages/7.x/apt stable InRelease
Hit:10 download.docker.com/linux/ubuntu jammy InRelease
Hit:11 in.archive.ubuntu.com/ubuntu jammy InRelease
Ign:12 example.com/repo stable InRelease
Hit:13 in.archive.ubuntu.com/ubuntu jammy-updates InRelease
Hit:14 in.archive.ubuntu.com/ubuntu jammy-backports InRelease
Hit:15 in.archive.ubuntu.com/ubuntu jammy-security InRelease
Ign:12 example.com/repo stable InRelease
Ign:12 example.com/repo stable InRelease
Err:12 example.com/repo stable InRelease
  500  Internal Server Error [IP: 93.184.215.14 80]
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done

2 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: file:/var/cuda-repo-ubuntu2004-11-6-local/Release.gpg: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

W: developer.download.nvidia.com/compute/cuda/repos/ubuntu2004/x86_64/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

W: download.docker.com/linux/ubuntu/dists/focal/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

W: download.docker.com/linux/ubuntu/dists/jammy/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

W: in.archive.ubuntu.com/ubuntu/dists/jammy/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
W: in.archive.ubuntu.com/ubuntu/dists/jammy-updates/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

W: in.archive.ubuntu.com/ubuntu/dists/jammy-backports/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

W: in archive.ubuntu.com/ubuntu/dists/jammy-security/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

W: Failed to fetch example.com/repo/dists/stable/InRelease  500  Internal Server Error [IP: 93.184.215.14 80]
W: Some index files failed to download. They have been ignored, or old ones used instead.

Answer 1

This is the problem:

(...)
Err:12 example.com/repo stable InRelease
  500  Internal Server Error [IP: 93.184.215.14 80]
(...)

You added "example.com/repo", possibly from some guide meant as an example but it isn't an actual repository, it's just a placeholder.

Remove it and apt should work as it is supposed (all the other warnings can be disregarded for now).

Answer 2

Removing the Non-Existent Repository Entry:

Open the Sources List File:

Use a text editor like nano or vim to edit the sources list file. The file is typically located at /etc/apt/sources.list or in a file within the /etc/apt/sources.list.d/ directory.

Identify and Remove the Problematic Entry:

Look for the line containing the entry for "example.com/repo" in the sources list file.

Delete or comment out this line by adding a "#" at the beginning of the line.

Save and Exit:

Save the changes you made to the file.

Close the text editor.

Update APT:

After removing the problematic entry, run the following command to update APT and refresh the package cache:

sudo apt update