Hello Francesca,
I see that your EndNote Online account is currently associated with “Universita di Milano Bicocca” as the institution.
This means that an Administrator for Universita di Milano Bicocca would need to be the one to post the style.
Provided this is what happened, the output style in the service should appear for you before long.
If that is an older institution where you are no longer, then please let us know, and we can remove that association so the EndNote Online service can re-affiliate with your new institution.
Thanks!
Steve