problem to add menu to top navigation bar

I am new to the Baskerville theme but like it a lot so I created a child theme, installed Simple CSS plugin and was able to change colours, move the navigation bar to the top etc. (thanks to this forum). However add a menu to the bar fails.

In my navigation bar there is one item “About”. To add the menu I go to Appearence – Menu , give a new name and push the button “create”. I adds the new name in the list but what to check or do next ?