If you are having a problem with starting Gunicorn, it normally means you either haven't activated your Virtual Environment, you haven't installed Gunicorn properly or you are missing dependencies. Below is a common list of issues with starting Gunicorn and solutions for each.
Ensure Gunicorn is Installed with pip not apt
When installing the Donation Store app, the setup.py file will do all of the configuration for you, but make sure you don't have Gunicorn installed using apt. If you have manually installed it that way, remove it and let the setup.py file install it for you.