When trying to write my email for enabling dropbox I couldn't write the @ symbol, I pressed Alt+2 and got 2. I have tried changing the keyboard layout into Spanish (first with no variant, then with "include dead tilde" variant). Each time that I go back to the keyboard layout menu the selected option is English (US).
I just deleted the English (US) layout, let the Spanish one and now I can write @. It would be good to say that here https://manual.cs50.net/appliance/19/#how_to_change_keyboard_layout