Read more link to copied text not working

function add_copyright_text() {
	if (is_single()) { ?>

<script type='text/javascript'>
function addLink() {
	if (
window.getSelection().containsNode(
document.getElementsByClassName('entry-content')[0], true)) {
    var body_element = document.getElementsByTagName('body')[0];
    var selection;
    selection = window.getSelection();
	var oldselection = selection
    var pagelink = "<br /><br /> Read more at WPBeginner: <?php the_title(); ?> <a href='<?php echo wp_get_shortlink(get_the_ID()); ?>'><?php echo wp_get_shortlink(get_the_ID()); ?></a>"; //Change this if you like
    var copy_text = selection + pagelink;
    var new_div = document.createElement('div');
	new_div.style.left='-99999px';
	new_div.style.position='absolute';

    body_element.appendChild(new_div );
    new_div.innerHTML = copy_text ;
    selection.selectAllChildren(new_div );
    window.setTimeout(function() {
        body_element.removeChild(new_div );
    },0);
}
}

document.oncopy = addLink;
</script>

<?php
}
}

add_action( 'wp_head', 'add_copyright_text');

https://www.wpbeginner.com/wp-tutorials/how-to-add-a-read-more-link-to-copied-text-in-wordpress/

I added this code at the functions.php file it gave me error then i enclosed it within <?php ?> then it didn’t but after that I tested it, it didn’t work.
But on another website which has a different theme it worked there