Be sure to install the dependencies first. You can do this from the terminal but be sure that you're in the correct virtual environment.
pip install pytest nbval
The commands that were used in this video are listed below.
pytest --nbval demo.ipynb
If you want to disable the warnings
pytest --nbval --disable-warnings demo.ipynb
If you don't care about cell outputs
pytest --nbval-lax --disable-warnings demo.ipynb
If you want the command line tool to ignore a cell simply add this comment to it;