Sorry, the correct output when I use "import nest" in the Subscriber is:
salvo@salvo-Aspire-GX-781:~/Desktop/ros_ws/src/pacchetto_ros/scripts$ rosrun pacchetto_ros sub_vrep.py Traceback (most recent call last): File "/home/salvo/Desktop/ros_ws/src/pacchetto_ros/scripts/sub_vrep.py", line 5, in <module> import nest File "/usr/lib/python3.6/dist-packages/nest/__init__.py", line 26, in <module> from . import ll_api # noqa File "/usr/lib/python3.6/dist-packages/nest/ll_api.py", line 72, in <module> from . import pynestkernel as kernel # noqa ImportError: dynamic module does not define init function (initpynestkernel)