Guys, please help me with this "Flexible Box Layout" problem!!!

Hi guys, I am trying to lay out my page using the "Flexible Box Layout" but I am having some irritating issue here, I've attached pics of the CSS file and the result, as you can see, for some reason, I get an "extra" box at the beginning of the navigation menu(I turned on the border left/right for better visual). Do you guys know why I am getting this issue please? Any suggestions are much much appreciated. Thank you very much. By the way, I am new to web coding.