If you encounter error "ModuleNotFoundError: No module named 'serial'" on your macOS when using Arduino,
it's perhaps Arduino is using Python 2 and not Python 3.
This Gist will allow you to install the pyserial module on Python 2
https://pypi.org/project/pyserial/#files
tar -xvf pyserial-x.x.tar.gz
cd pyserial-x.x.tar.gz
sudo python2 setup.py install