Error installing `bullseye-backports` when trying to install for Development

Support Self-hosting
1

Following the guide Install Discourse on Ubuntu for Development, on a new virtual machine with Ubuntu 20.04 I get this error:

E: The value 'bullseye-backports' is invalid for APT::Default-Release as such a release is not available in the sources failed

when I try to run:

bash <(wget -qO- https://raw.githubusercontent.com/discourse/install-rails/master/linux)

I get the same error at the same step when trying to follow the guide Install Discourse on Windows 10 for development

2

The only working installation guide seems to be

https://raw.githubusercontent.com/discourse/install-rails/master/linux seems to be outdated.
It installs ruby 3.1.3 and node 16, while the current base image uses ruby 3.2.1 and node 18.

I got it running on Debian 11 but got other errors in the follow-up.
Switching to the dockerized dev environment solved most problems.