In this example, you need to install box2d. For some reason, you have to install box2d from source.
# Install swig in proper way to your OS.
$ brew install swig
# Install box2d from source.
$ git clone https://github.com/pybox2d/pybox2d
$ cd pybox2d
$ python setup.py clean
$ python setup.py build
$ python setup.py install
$ cd ../ && rm -Rf pybox2d
Run example
$ cd examples/ppo_bipedalwalker_v2
$ python main.py