引导3下拉菜单中心对齐不工作

I am not able to align the dropdown to the center of the parent on which the dropdown is opened with hover. I have tried to text-align: center for the entire .nav .navbar li elements, and it doesn't work. I have tried to set margin and left: 50% for the entire ul.dropdown-menu that is the dropdown and it doesn't work. Here is the html code:I am not able to align the dropdown to the cent