Paolo,
I notice that your app has its screens at the same level as the navigator rather than being within the navigator. The easiest way to ensure that a screen is within a navigator is to make sure that when you add it that you drag the screen right on top of the navigator in the component tree. You should see the left border of the navigator turn blue while you’re dragged the screen right over the navigator. Once you’ve properly dragged in your screens (and labels) the component tree should look something like this:
Here’s an example project:
Thunkable
We probably need to do more UI work to make this easier!
Hope this helps.
And, yes, #Thunkable ✕ (Beta) is the appropriate Category for asking this kind of question.
-Mark