I just installed this yesterday on a clean CentOS 7 & Nethserver 7.9.
Installation was literally 2 setps on Nethserver
Add OpenProject repo
sudo wget -O /etc/yum.repos.d/openproject.repo \
https://dl.packager.io/srv/opf/openproject/stable/11/installer/el/7.repo
Install Open Project
sudo yum install openproject