macOS
Open System Settings, then General, then Sharing, and turn on Remote Login. Allow your own user.
Ubuntu or Debian
sudo apt install openssh-server sudo systemctl enable --now ssh
Fedora
sudo dnf install openssh-server sudo systemctl enable --now sshd
Arch
sudo pacman -S openssh sudo systemctl enable --now sshd
Check
From the machine itself:
ssh localhost
It should ask for your password and give you a prompt. Type exit to leave.
If someone turned off stream-local forwarding (AllowStreamLocalForwarding no in /etc/ssh/sshd_config), turn it back on or make sure python3 is installed; the app falls back to it.