This part of the documentation covers the ways to run PatZilla. Please read this section carefully.
If the MongoDB database already is running on your machine, you can skip this step.
You can either install and run it system-wide on Debian GNU/Linux:
apt install mongodb-clients mongodb-server
… or run it ad-hoc in the current working directory:
mkdir -p ./var/lib/mongodb mongod --dbpath=./var/lib/mongodb --smallfiles