I am trying to instantiate a class in one JavaScript file in another HTML file but I keep on getting this error.

Here is the code for the JavaScript file:

class Puzzle {
  constructor(fenStart, pgnEnd) {
    this.fenStart = fenStart;
    this.pgnEnd = pgnEnd;
  }
}

And here is the code for the HTML. It should be noted that I am also using chessboard.js and chess.js and that everything is saved in the same folder.

  
    
    
  
  
  
  
  

  
PGN: 
  
  
  
  

    
  

What is causing this error and how can I fix it?