Same problem on Debian 11.
After half a day's work I managed to work out this solution:

sudo apt update
sudo apt install -y build-essential
sudo apt install -y python
sudo apt install -y python-dev libldap2-dev libsasl2-dev ldap-utils tox lcov valgrind


python get-pip.py
pip install ajenti


sudo apt install -y curl
curl https://bootstrap.pypa.io/pip/2.7/get-pip.py -o get-pip.py

sudo apt -y install gnupg2
wget http://repo.ajenti.org/debian/key -O- | sudo apt-key add -
echo "deb http://repo.ajenti.org/debian main main debian" | sudo tee /etc/apt/sources.list.d/ajenti.list

sudo apt update
sudo apt download ajenti
sudo dpkg -i --force-depends ajenti_1.2.23.13_all.deb

Problem after installing ajenti like this is that `apt` thinks it is now holding broken dependencies. It always asks you to run `apt --fix-broken install`, which would then uninstall ajenti again.