IDLE uses Python's tcl/tk interface module, called 'tkinter'. Tcl is a separate language and tk is its graphical user interface. This is most likely the problem. Did you follow the advice at
https://www.python.org/download/mac/tcltk/
to install tcl/tk 8.5.18? Make sure you read the whole page.
To test better, run 'import tkinter' at Python's interactive '>>>' prompt. Any error message? If that works, run 'python3.6 -m idlelib' and/or 'python -m idlelib.idle' at the terminal '...$' prompt. Does that display an error message?