Debian 11 Bullseye 64bits

Installation instructions

Logs Download 
wget -qO- https://dl.packager.io/srv/zammad/zammad/key | sudo apt-key add -
sudo wget -O /etc/apt/sources.list.d/zammad.list \
  https://dl.packager.io/srv/zammad/zammad/develop/installer/debian/11.repo
sudo apt-get update
sudo apt-get install zammad

Note: You might need to apt-get install wget apt-transport-https for the above instructions to work.

More details can be found at https://docs.zammad.org .