IntelliJ IDEA - how to enable keyboard shortcuts
Maybe this shortcut is not assigned in IntelliJ (at least not for the italian keymap) or a different shortcut is mapped to block comments.

First you can lookup if this shortcut is mapped (Preferences -> Keymap (or search for keymap). The keymap preferences dialog contains a search field (top right). If you search for 'block', you will get all mappings.

You may assign your desired shortcuts (double click on "Comment with Block Comment") and IntelliJ even warns you if your shortcut is already assigned.

Additionally there is an open issue in their issue tracker for "screwed up shortcuts" for many non-US keyboard layouts (see https://youtrack.jetbrains.com/issue/IDEA-167193)