The problem of not adding an event to the button – Blancer.com Tutorials and projects

I am studying html, css, and js.

To add a click event to the arrow button of flickity,

I used addEventListener. However, the following error occurred.

TypeError: Cannot read properties of null (reading 'addEventListener')
    at index.js:3:6

I am still a beginner, so I do not know what is wrong.

html

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="style.css">
    <link rel="stylesheet" href="https://unpkg.com/flickity@2/dist/flickity.min.css">
</head>
<body>
    <h1>My Calendar</h1>

    <div class="header">
        <div class="carousel" data-flickity='{ "pageDots": false }'>
            <div class="carousel-cell">1</div>
            <div class="carousel-cell">3</div>
            <div class="carousel-cell">4</div>
            <div class="carousel-cell">5</div>
        </div>
    </div>

    <div class="container">
        
    </div>
    <script src="https://unpkg.com/flickity@2/dist/flickity.pkgd.min.js"></script>
    <script src="index.js"></script>
</body>

css

.carousel-cell {
    width: 100%;
    font-size: 48px;
    background: transparent;
    text-align: center;
}


.carousel .flickity-prev-next-button {
    width: 100px;
    height: 50px;
    background: transparent;
    border-radius: 0;
    border: none;
    box-shadow: none;
  }

 .carousel .flickity-prev-next-button.previous {
    left: 250px;
}

.carousel .flickity-prev-next-button.next {
    right: 250px;    
}

javascript

const btn = document.querySelector(".carousel .flickity-prev-next-button");


btn.addEventListener("click", () => {
    console.log("Hello")
});