Error: Python GUI toolkit for qt5 is not installed

I have a problem with PyCall. I want to use Qt5 backend (in Juno). I thus change the default backend (tk) doing:

using PyCall

And I get the following message: “Python GUI toolkit for qt5 is not installed”. However, Qt5 is installed in Python. It is in fact its default backend - the following lines in Python:

import matplotlib.pyplot as plt

give “Qt5Agg”.
Any one can help?