When I try to run the command sudo apt-add-repository 'deb https://apt.kitware.com/ubuntu/ bionic main in the setup instructions, I run into this error:
Hit:1 http://us.archive.ubuntu.com/ubuntu focal InRelease
Get:2 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Get:3 http://us.archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Hit:4 http://packages.microsoft.com/repos/code stable InRelease
Hit:5 https://nvidia.github.io/libnvidia-container/stable/ubuntu18.04/amd64 InRelease
Hit:6 https://nvidia.github.io/nvidia-container-runtime/stable/ubuntu18.04/amd64 InRelease
Hit:7 https://nvidia.github.io/nvidia-docker/ubuntu18.04/amd64 InRelease
Hit:8 https://apt.kitware.com/ubuntu bionic InRelease
Hit:9 http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu focal InRelease
Get:10 https://download.docker.com/linux/ubuntu focal InRelease [52.1 kB]
Get:11 http://us.archive.ubuntu.com/ubuntu focal-backports InRelease [101 kB]
Err:12 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease
402 Payment Required [IP: 2600:1f1c:2e5:6901:48c1:8c0c:948d:3ce7 443]
Hit:13 http://ppa.launchpad.net/openjdk-r/ppa/ubuntu focal InRelease
Reading package lists… Done
E: Failed to fetch https://packagecloud.io/slacktechnologies/slack/debian/dists/jessie/InRelease 402 Payment Required [IP: 2600:1f1c:2e5:6901:48c1:8c0c:948d:3ce7 443]
E: The repository ‘https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease’ is no longer signed.
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.