I think we would need to see your HTML and CSS in order to give you help without just guessing. But if I had to guess, a common problem people run into with this test is that the browser adds default margins to the body and you need to remove those in order to get the nav menu all the way to the top of the viewport.
To display your code in here you need to wrap it in triple back ticks. On a line by itself type three back ticks. Then on the first line below the three back ticks paste in your code. Then below your code on a new line type three more back ticks. The back tick on my keyboard is in the upper left just above the Tab key and below the Esc key. You may also be able to use Ctrl+e to automatically give you the triple back ticks while you are typing in the this editor and the cursor is on a line by itself. Alternatively, with the cursor on a line by itself, you can use the </> button above the editor to add the triple back ticks.