pgadmin4: no module flask

Hi all,
still trying to get pgAdmin4 running on my system. When I launch it I got:

./runtime/pgAdmin4
Webapp path: "/home/luca/tmp/pgadmin4/web/pgAdmin4.py"
Traceback (most recent call last):
File "/home/luca/tmp/pgadmin4/web/pgAdmin4.py", line 24, in <module>
from pgadmin import create_app
File "/home/luca/tmp/pgadmin4/web/pgadmin/__init__.py", line 13, in <module>
from flask import Flask, abort, request, current_app
ImportError: No module named flask
"Failed to launch the application server, server thread exiting."

and a configuration window keep asking me the python path and the
application path (and I don't know what to specify for it).
I've installed flask via pip and via apt, am I missing something?

Thanks,
Luca