You can only skip forward if you've already played the game through. The 'skip' button at the bottom of the dialogue box will be clickable after this.
I don't want to skip any text, I'd just like to turn on the auto so the text boxes will play by themselves? I assumed since it says "Auto Forward" in the preferences that this is something the game could do? I don't know a lot about the functions of Ren'Py, sorry. Anyway, if it is possible to turn this on it would be nice for my carpal tunnel.