I created a bug report and was told it really wasn't a bug to use the appimage available. I pointed out the website/documentation was wrong and was told to report that here.

The bug report:
https://bugs.kde.org/show_bug.cgi?id=407476


SUMMARY
I'm unable to add the Kirtta repository on Ubuntu 19.04 Disco Dingo.


STEPS TO REPRODUCE
1. Enter "sudo add-apt-repository ppa:kritalime/ppa" into terminal.
2. Get welcome screen and press enter to add repository.
3. See error from package manager.


OBSERVED RESULT

Err:13 http://ppa.launchpad.net/kritalime/ppa/ubuntu disco Release
404 Not Found [IP: 91.189.95.83 80]
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/kritalime/ppa/ubuntu disco Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.


EXPECTED RESULT
No error when adding the Kritta reository to Ubuntu 19.04 Disco Dingo.

This technically should be reported to Ubuntu, or more precisely, the PPA's maintainer.
It looks like there simply were no packages in the Disco section of the PPA until a few hours ago. It works fine for me just now, so I suggest running apt update again, and try installing krita-testing, as that is the only thing available for Disco from there.19.04 has the latest stable krita 4.1.7 already, hence the lack of packages in the PPA for 19.04.

There is no bug here, really.

https://launchpad.net/~kritalime/+archi ... lter=disco

According to the PPA I have reported it to the maintainers. "If you have Ubuntu 16.10 or newer, please use Krita from this repository, this is an officially recommended way alongside AppImage builds."
https://launchpad.net/~kritalime/+archive/ubuntu/ppa


"A rose by any other name would smell as sweet", William Shakespeare. If I can't follow the official documentation to install a piece of software, I would call that a rose. FYI, those testing packages were created after I reported the rose.

They probably need to update that blurb a little, maybe.

Anyway, it is still true that the current *buntu release has the latest Krita at the moment, so there is little use in creating a redundant package, at least not until there is a new version released.

But, the problem has since been fixed as there is now a Disco repo created since you saw the error, so no one should see that happen now.
There just won't be a stable Krita package in it right now, only a pre-release.