How do I change the text content of a div for a comment validation error message?

if your above Javascript is working fine and you want to insert that into a page.

Kindly add the following code to your functions.php.
I have tested this code it’s working fine.

function handle_comment_error() {
?>
    <script>
    document.getElementById("comment-error").innerHTML = "Please enter your comment.";
    </script>
<?php
 }add_action('wp_head', 'handle_comment_error');

Let me know if you face any issues.