Hamburger Drop Down Not Responding To Click Event

You have quite a few issues in your js. First of all, you need to make sure you are keeping variable names consistent. (You were using ddc instead of x in places. Furthermore, make sure that strings are inside of quotes, otherwise, they are treated as variables.

Fixed code:

<!DOCTYPE html>
   <html lang="en">
      <head>
         <title>MY CSS WEBSITE</title>
         <link rel="stylesheet" href="style.css">
      </head>
      <body>
         <nav>
            <ul class="topnav" id="dropdownClick">
            
              <li>
                  <a href="#home">Home</a>
              </li>
              <li>
                  <a href="#about">About</a>
              </li>
              <li>
                  <a href="#news">News</a>
              </li>
              <li>
                  <a href="#contact">Contact</a>
              </li>
              <li class="topnav-right">
                  <a href="#signUp">Sign Up</a>
              </li>
              <li class="topnav-right">
                  <a href="#signIn">Sign In</a>
              </li>
              <li class="dropdownIcon">
                  <a href="javascript:dropdownMenu()">☰</a>
              </li>
          </ul>
        </nav>     
    <script> 
        function dropdownMenu(){
            console.log("hello");
            var x = document.getElementById("dropdownClick");
            if(x.className === "topnav"){
                x.className += " responsive";
            }
            /*change topnav to topnav.responsive*/
            else{
                x.className = "topnav";
            }
        }

    </script>
</body>