there are a lot of problems in your code. main problem is you are not creating and closing the tag. img and input tag is self closing with '/>'.
<link rel="stylesheet" href="styles.css">
<header id="header">
<img id="header-img" src="https://placehold.co/40" alt="" />
<nav id="nav-bar">
<ul>
<li> <a class="nav-link" href="#Features">Features</a>
</li>
<li> <a class="nav-link" href="#How-It-Works">How It Works</a>
</li>
<li> <a class="nav-link" href="#Pricing">Pricing</a>
</li>
</ul>
</nav>
</header>
<main>
<video id="video" src="https://youtu.be/y8Yv4pnO7qc"></video>
<form id="form" action="https://www.freecodecamp.com/email-submit"> </form>
<input id="email" name="email" placeholder="Enter your email address" type= "email" />
<input id="submit" type="submit" />
<button id="submit"> submit</button>
</main>