Debian 11 Bullseye 64bits

Installation instructions

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

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