apt update fails: E: The repository 'http://ppa.launchpad.net/elementary-os/stable/ubuntu disco Release' does not have a Release file.

[Ralf Mardorf]

On Sat, 19 Oct 2019 09:12:29 +0200, robert rottermann wrote:
>Err:13 http://ppa.launchpad.net/elementary-os/stable/ubuntu disco

Hi,

the third-party repository you added fails, so you need to contact
support of this third party repository.

Take a look with a browser, e.g. Firefox.

firefox http://ppa.launchpad.net/elementary-os/stable/ubuntu/dists/

This third-party repository does not support Ubuntu disco (19.04).

Remove the repository from your sources list. Stay with the official
Ubuntu repositories.

You could use ppa-purge to remove this unsupported third-party
repository.

https://packages.ubuntu.com/disco/ppa-purge
http://manpages.ubuntu.com/manpages/disco/man1/ppa-purge.1.html

Regards,
Ralf