The fastjet directory in this repository is already compiled. It can be loaded in the python scripts by adding it's path to bash_profile:
LD_LIBRARY_PATH=$LD_LIBRARY_PATH:'pwd'/fastjet-install/lib
PYTHONPATH=$PYTHONPATH:'pwd'/fastjet-install/lib/python3.7/site-packages
In the above lines you need to edit the 'pwd' to the directory path.
In case there is some error with the above procedure you can re-build the fastjet again using:
./configure --prefix='pwd'/../fastjet-install --enable-pyext
make
make check
make install
For information on the LHC Olympics 2020, see this website.