Hi, i need that the user "operatore" can edit the message in frontend. Can you help me?
In Joomla by default only Admin and super user can able to edit the articles.
If you want to add another user to edit the content article you can change the user group access to Editor level in that case he can able to edit/add articles on website.
If you only want to give access via frontend you can create a new Joomla user group and give them access via ACL settings : https://docs.joomla.org/J3.x:Access_Control_List_Tutorial
Hope it helps.