Cheers, thank you!
I feel it was unnecessary for you to do so, but I thank you nonetheless. I neglected to echo & curl the URL for the key to apt-key add. If you don’t mind I’m going to post the answer to hopefully alleviate seeing these threads in the future and to have a written “solve” for the question, as I hate a lack of apparent closure.
FOR THOSE THAT ARE HAVING THIS PROBLEM WITH UBUNTU NOT HAVING A SIGNED CERTIFICATE FOR PLEX HERE IS THE FIX
If you are getting the error:
Err:6 https://downloads.plex.tv/repo/deb public InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 97203C7B3ADCA79D
Reading package lists... Done
W: GPG error: https://downloads.plex.tv/repo/deb public InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 97203C7B3ADCA79D
E: The repository 'https://downloads.plex.tv/repo/deb public InRelease' is not signed.
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.
user@hostname:~$
Presuming you installed from the .deb file downloaded from the Plex site, gold and glory await you below! Okay not really, but below is your remedy:
- You need to get the URL for the public repo into /etc/apt/sources.list.d/plexmediaserver.list - so TL;DR version: just copy/paste what I’ve got below in the quote/codeblocks and you’re good to go! If you want to understand what these commands do then read the paragraph below (I’ll do the same for the other steps).
echo deb https://downloads.plex.tv/repo/deb public main | sudo tee /etc/apt/sources.list.d/plexmediaserver.list
Now for those of you trying to learn more about and tweaking Linux (likely Ubuntu, maybe Debian if you’re wanting to get a little “risqué”. Anyway simply with your presence here I can say it’s safe to say that you enjoy messing with things (computers obviously, probably cars, hookers - whatever) and understand how they work. So, I’ll go into more depth about exactly what you’re doing so you’ll know for the future. And I mean at this point let’s be frank, you’ve either opted for the TL;DR version and you’re probably not even reading this and are just gonna plow forward and copy/paste what I put in the blocks. Better hope I’m not a nefarious asshole 
). Kidding, or am I? 
. Alright, enough dicking around, now’s it’s time to understand what exactly these commands you’re typing (or copying) DO.
For those that are still with me: echo is a program (okay it’s really just a caveman shell command) that copies/feeds back/displays a line of text, specifically what you type after the “echo” command. Seriously, type echo waffles and see what it spits back. Last time I checked “waffles” wasn’t a Linux command - it just outputs what you typed. So in this case it’s outputting back the URL for the Plex repo along with the prefix of deb and suffixes of “public” and “main”. Deb just means that this package repo distributes software packages as precompiled binaries in .deb file format (which is natively understood by Debian distros (of which Ubuntu is a member) - there’s other Linux package formats, but I like .deb the best (and thanks to Plex devs for making a .deb installer!), but I’m SERIOUSLY off the path here.
Anyway, you output the repo for the public repo back to the command line with echo, then in that same single line command, you see there’s an additional one for “tee” separated by a pipe ( | ). This tells the computer that it’s a new command, hence why after the pipe we have sudo tee. Tee reads standard input (what was just output with echo), then takes it and writes it to the Plex sources.list file. We know that because tee gave the location to write to. For more info on these two commands read their man pages. Just type man echo & man tee.
2: Add the signed public key to said repo that you just added:
curl https://downloads.plex.tv/plex-keys/PlexSign.key | sudo apt-key add -
‘curl’ feeds URLs into commands to then be run on your machine. You’ll see a lot of ‘curl’ with github stuff. For more info on ‘curl’ type man curl.
3: Update (I added upgrade to the end so it’ll upgrade any of the packages that might have fallen by the wayside while you were figuring this out, so if you just want to update remove everything after “&&”
sudo apt-get update && sudo apt-get upgrade
Here’s a little bit of knowledge that took me having quite a few years of running Linux under my belt before I understood the differences between && ; , | when it comes to using them in a terminal. Yes they can all be used to run multiple commands when just entering one line on the terminal, but there’s subtle differences.
; - makes commands executed sequentially - the next one starts after the first one is done.
&& - Similar to ; The command following && is executed ONLY if the first succeeds.
| - The pipe essentially connects the two commands it’s in the middle of. You couldn’t use any of the above control/redirect operators on the echo/tee example above that we used here. The reason why is because echo would have left the output there, but none of the above listed would “pick it up” and use it for command 2 - tee in this case.
I hope that explained it - if not man bash has more info.
4: Profit.
I feel ridiculous typing my own solution which I will then tag as the solution to my problem after this, but I REALLY hope it’ll have helped someone in the future. Mostly cuz I spent WAY too long editing this and adding the explanations. So - seriously if this helped you, say something here or drop me a message if it’s closed or something. I wanna know if:
A: How I explained it all worked for you
and
B: That it actually helped someone. I seriously spent way too much time on this for it to have gone to waste 
Also the attached link is the page I somehow missed in my searches early this morning is the page which I figured out is the fix to my (and yours dear reader) problem.
https://support.plex.tv/articles/235974187-enable-repository-updating-for-supported-linux-server-distributions/
Thanks again @ChuckPa for doing a (likely) unnecessary cage rattle on my behalf. If this somehow isn’t the fix for this issue, rest assured that I’ll be back in here whining at my own laughable ignorance.
Hope it helps!
Cheers,
Grant
