You need to install a GUI toolkit for so that Matplotlib can display graphics (outside of Jupyter, where the browser handles display). For example, if you install the pyqt package then PyPlot should automatically use the qt5agg backend.
(From your other post, you are using a custom Python installation — things would probably go more smoothly if you allowed Julia to install its own Python via the Conda package, in which case it could install Python packages for you.)