DROPDOWN TOGGLE in header doesn´t work

Hi everyone,

I want to know, how I can create in header navbar site navigation DROPDOWN TOGGLE, because it doesn´t work. I made Pages under the Parent Page in header Main Menu, but they don´t drop down and I can´t find them. Please, if it´s possible, help me how can I create them, what to do to change it? Thank you very much, yours sincerely

Andrea