404 on attempting to install Musescore 3 on Linux

I'm running XFCE on my Chromebook using crouton (though I don't know the exact operating system or architecture or whatever; I'm no Linux wiz), trying to install Musescore 3. Musescore 2 installed just fine, though I didn't realize it was Musescore 2 until I tried to load a score on my main computer - apparently my powers of observation are completely absent. Anyway, that's why I tried to install version 3.
I followed the directions on the package page (I'm using ppa):

>sudo add-apt-repository ppa:mscore-ubuntu/mscore3-stable

No errors here, everything seems ship-shape.

>sudo apt-get update

Here's where the problem occurs. All the localizations come in fine, then I see:

>Err:8 http://ppa.launchpad.net/mscore-ubuntu/mscore3-stable/ubuntu xenial/main amd64 Packages
> 404 Not Found [IP: 91.189.95.83 80]

And that's a problem, as the lines at the end of it say:

>W: The repository 'http://ppa.launchpad.net/mscore-ubuntu/mscore3-stable/ubuntu xenial Release' does not have a Release file.
>N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
>N: See apt-secure(8) manpage for repository creation and user configuration details.
>E: Failed to fetch http://ppa.launchpad.net/mscore-ubuntu/mscore3-stable/ubuntu/dists/xeni… 404 Not Found [IP: 91.189.95.83 80]
>E: Some index files failed to download. They have been ignored, or old ones used instead.

So there's no release file, huh?
So I try the next step,

>sudo apt-get install musescore3

which should be right. Unsurprisingly, I get the error

>E: Unable to locate package musescore3

I need some help, or insight. Am I do something wrong? Is it my operating system? Again, Musescore 2 worked fine. It even ran, was able to connect to the sound card (though the MIDI keyboard had trouble - but that's a different topic entirely), and load and save files. Is it something on Musescore's end? Did they forget to upload a release file?
Something worth noting is I tried to install the nightly instead. This worked fine, but - the nightly was 2.1, not any future version. So I wager the Linux versions are just massively behind...despite mscore3 being listed on the package page.