I just wanted to add a small follow up comment because I remember being young and copy-pasting commands into Linux and eventually getting really frustrated. Therefore, he’s a (brief) explanation of the commands:

1. curl is just an open source tool for making Web requests from the command line. It’s a great tool to have in general.
2. https://repo.jellyfin.org/install-debuntu.sh the URL of a shell script from repo.jellyfin.org (Jellyfin’s official website)

What is a shell script? It’s a script that runs a whole bunch of commands by itself, so you don’t have to copy-paste them from the internet. Basically the official Jellyfin people in this case made a file with all of the commands the computer needs to run to install the package. This is great because it means the people who made Jellyfin tested these commands and they’re responsible for keeping it up to date if anything changes.

| bash The ‘pipe’ or | symbol in Linux is a cool Unix philosophy of ‘connecting’ programs together. You run one program, and tell it to pass the results to another program. In this case, you’re telling curl to download the script at https://repo.jellyfin.org/install-debuntu.sh and then passing that file to bash (which is the shell program in the terminal that runs commands) and to run it as sudo or ‘super-user’.

Hope this was helpful. The last thing you should know is the command you probably copy-pasted before made you add a source to the /etc/apt/sources files, which are basically just a list of sources for apt, the package manager to download from, and since the command was wrong or outdated, apt is complaining that the Jellyfin source was not found.