Flatpak Repo
 

I've had trouble with setting this up, doing updates, etc for a fresh install. I'm getting the 404 error for flatpak repo that says it does not have a release file. I did research, but nothing has helped.

Cosmic Cuttlefish is not in the lists of distributions.

I don't understand what you mean. What do I need to do to fix it?

According to your sources.list, you are running Ubuntu 18.10 Cosmic Cuttlefish, which is a short term OS only supported till July 2019, it's kind of a "Beta" next generation Ubuntu. The PPA flatpak designer you are trying to install flatpak from is not willing to take the time to port his software to an OS that is not official yet.
You should either install Ubuntu 18.04.1 woodchuck which he has software ported too, or take a chance and change out "cosmic" for for "bionic" in your sources.list, it may work, then again it may wreck your installation which might force you to just install 18.04.1 in the end.

Example of sources.list change from original to "take a chance":

^ in other words, unless you're 100% sure you know what you're doing, you should stick to Ubuntu's LTS releases.
https://wiki.ubuntu.com/Releases

i will add this little piece of unsolicited opinion/advice:

1. there's only one way to use ubuntu, that is: don't attempt to tinker with it or adjust certain aspects of it to fit your needs (beyond some very basic things).

2. under the hood, ubuntu does some things differently than other Linuxes, e.g. increasingly using snaps or flatpak whatchamacallits instead of good ol' package management. this can become particularly confusing if you don't know about it, ask for advice, and get advice that applies to classic package management.

there's other beginner friendly distro's out there.

just saying.