Having trouble linking my .css file to my html file [duplicate]

I am having trouble linking my CSS file to my HTML file despite being in the same folder:

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CSS Intro</title>
    <link rel="stylesheet" type="text/css" href="index-css.css">
</head>

<style>
    h2 {
        color: palevioletred;
    }
</style>



<body>

    <!--Inline styles - BAD! -->
    <!-- <h1 style="color: purple"> Hello World </h1>
    <button style="background-color : palegreen">I AM BUTTON</button> -->
    <h2>I AM A NEW COLOUR</h2>

</body>

</html>

<! — css file: index-css.css –>

h2 {
    color: indigo;
}

<! — so I’d like to know why the css file is not properly integrated with the html file despite both files being in the same folder? I am brand new to coding and just finished with html lesson and now started css concepts and learning css before going into JavaScript. Can someone explain to me what’s going on? –>