To run the server it is highly recommended that you install virtualenv

Install virtualenv pip install virtualenv

Find our where your python 3.5+ installation is located: which python3 (or which python)

virtualenv -p /usr/local/bin/python3 pizza-venv

Now you can activate the virtual python environment by issuing the following command source pizza-venv/bin/activate

You should see parentheses at the beginning of your prompt. E.g:

~/Documents/code/private/pizza (master) $ pwd
/Users/felixnovovic/Documents/code/private/pizza
~/Documents/code/private/pizza (master) $ source pizza-venv/bin/activate
(pizza-venv) ~/Documents/code/private/pizza (master) $

Install dependencies: pip install -r requirements.txt

The above install command will not mess with your existing python installation if you are running in venv.

Run the app:

FLASK_APP=app.py flask run

If everything has been configured correctly you should be able to view the site at http://localhost:5000