"Your #nav-bar should always be at the top of the viewport?

I am doing the “Build a product landing page” project which is project #3 in Legacy Responsive Web Design

I already added my code for this:

header {
position: fixed;
top: 0px;
width: 100%;
height: 20px;
display: flex;
justify-content: space-between;
align-items: center;
background-color: black;
opacity: 0.8;
}

why is it not working?