Installing pygal under Ubuntu 18.04

[Gribouillis]

Is it posible to change the version of Python that the IDLE is runing?

You seem to have many versions of python installed. Each version comes with its own idle and its own pip. Assuming the executable for python 3.6 is /usr/bin/python3.6, you can launch idle from a terminal by typing

Output:
/usr/bin/python3.6 -m idlelib

You could also install cffi for your python 3.7. Find the executable by typing

import sys
print(sys.executable)

in your python 3.7's IDLE. Then, if this executable is for example /usr/local/bin/python3.7, you can install cffi for python 3.7 by typing in a terminal

Output:
sudo /usr/local/bin/python3.7 -m pip install cffi