Hi,
The issue is that you are using a ROOT installation that was built for Python2. Therefore, it works when you use the Python2 interpreter, but throws an error when you use Python3. Current PyROOT can only be built for one Python version, we have removed that limitation in a new PyROOT we are working on but it is still present in the current one.
If you want to use Python3, you will need to build ROOT with Python3, by specifying the right parameters of CMake:
https://root.cern.ch/building-root
in particular PYTHON_EXECUTABLE .