We might want to see the top-level config.log, not just the last few lines of it.
The following lines from the pplpy log:
Running post-install script for pplpy-0.8.6.
/bin/sh: /Users/youngsu/Downloads/sage/sage-9.4/local/bin/sphinx-build: /Users/youngsu/Downloads/sage-9.4/local/bin/python3: bad interpreter: No such file or directory
suggest that something went wrong with building Python3.
(which you can use from the system, and not build, normally speaking)