There does not appear to be any problem based on error_messages.txt. All those messages are printed by SNAP biophysical processor regardless of how it’s run and do not seem to affect the output.
The graph does not depend on any Python version but for running the Sen-ET scripts you need to have setup SNAP to work with Python 3.6. See section 3.2 of the user manual. The plugin was developed and tested with SNAP 7 but some users on the forum appear to be using it with SNAP 8 as well.