I see this was kind of reported here too: Unable to locate the package sensu-go-backend - #2 by JustinToo
The official install method of using the scripted solution from packagecloud also results in the same failed install:
# Add the Sensu repository
curl -s https://packagecloud.io/install/repositories/sensu/stable/script.deb.sh | sudo bash
# Install the sensu-go-agent package
sudo apt-get install sensu-go-agent
example:
root@machine345:~# curl -s https://packagecloud.io/install/repositories/sensu/stable/script.deb.sh | sudo bash
Detected operating system as Ubuntu/noble.
Checking for curl...
Detected curl...
Checking for gpg...
Detected gpg...
Detected apt version as 2.7.14
Running apt-get update... done.
Installing apt-transport-https... done.
Installing /etc/apt/sources.list.d/sensu_stable.list...done.
Importing packagecloud gpg key... Packagecloud gpg key imported to /etc/apt/keyrings/sensu_stable-archive-keyring.gpg
done.
Running apt-get update... done.
The repository is setup! You can now install packages.
root@machine345:~# apt update
Hit:1 https://download.docker.com/linux/ubuntu noble InRelease
Hit:2 http://security.ubuntu.com/ubuntu noble-security InRelease
Hit:3 http://archive.ubuntu.com/ubuntu noble InRelease
Hit:4 http://archive.ubuntu.com/ubuntu noble-updates InRelease
Hit:5 https://ppa.launchpadcontent.net/ansible/ansible/ubuntu noble InRelease
Hit:6 http://archive.ubuntu.com/ubuntu noble-backports InRelease
Ign:7 https://packagecloud.io/sensu/stable/ubuntu noble InRelease
Err:8 https://packagecloud.io/sensu/stable/ubuntu noble Release
404 Not Found [IP: 52.52.87.14 443]
Reading package lists... Done
E: The repository 'https://packagecloud.io/sensu/stable/ubuntu noble Release' does not have a Release file.
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.
Source: Install Sensu - Sensu Docs