According to the log: Additionally, the navbar should contain link (<a>) elements with the class of "nav-link". There should be one for every element with the class "main-section".
Let’s look at your code now:
<!-- Your nav with the id navbar and the header inside, this ok -->
<nav id="navbar">
<header>Front-End Basics</header>
</nav>
<!-- Next of your code -->
<br>
<br>
<!-- You created the links with the class nav-link, this is good, the issue is that they are out of the element with the navbar id -->
<a class="nav-link" href="#Introduction">Introduction</a>
<a class="nav-link" href="#What_you_already_know">What you already know</a>
<a class="nav-link" href="#HTML">HTML</a>
<a class="nav-link" href="#CSS">CSS</a>
<a class="nav-link" href="#JavaScript">JavaScript</a>
I put comment directly into the code. Do you understand your issue?