He had added a Jellyfin repo to his apt sources.list file for some reason, which is weird and likely not the right way to do it these days. But it might have been in the past, so it could be OP was following some obsolete procedure (or one AI-hallucinated from an obsolete procedure).

After realizing that OP was completely going about it the wrong way, the guy you replied to just looked up the correct way and relayed that to him.

See also: https://wiki.debian.org/DontBreakDebian

Not OP, but it was very simple if you have already seen that error.

First of all, there is one single easily parsable error.

https://repo.jellyfin.org/debian produced a 404 error, thus the URL is invalid.

Let’s ignore why it’s invalid for a second.

This error happens after apt update, thus we can deduce the following:

• It’s supposed to be an apt repository URL (To experienced users, it effectively looks like a repository URL)
• This repository URL does not work
• As in 99% of cases, this URL is likely located in a configuration file in the standard location, /etc/apt/sources.list.d/

Back to why it’s invalid, maybe it used to be valid in the past, or there is a temporary server error, this can be verified with the official documentation.

If the documentation does not mention this repository URL, then it’s a mistake to use it.

This is a good moment to google this URL and find out why/which guide tells you to use it, and to analyze which steps they made you take.

From there, reverse those steps.

Even if you hadn’t found this guide, you can be sure that by looking into /etc/apt/sources.list.d you would’ve found that file containing that URL, simply removing the file or URL would’ve removed the error.

Lastly, you look for either the official documentation, or a more reliable guide.