Changes to functions.php not working

Closing PHP tag and then opening in the next line:

Whenever there is something (even space, tab, newline etc.) after a closing PHP tag ?> or before an opening PHP tag <?php, PHP engine considers it as output.

So the way your code is, it’ll generate some unnecessary output, that often throws error or warning like:

Header already sent …

So you must make sure there is no unintended output, even if you need multiple opening and closing PHP tags.

Closing PHP Tag ?>

PHP doesn’t need the closing PHP tag ?>.
If the file ends while within the scope of any opening PHP tag <?php, PHP considers the file ending to be the end of the PHP tag.

CODE fix:

So in that light, your CODE will be like:

<?php 
    add_action( 'wp_enqueue_scripts', 'sydney_child_enqueue_styles' );
    function sydney_child_enqueue_styles() {
        wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' ); 
    }

    // BEGIN ENQUEUE PARENT ACTION
    // AUTO GENERATED - Do not modify or remove comment markers above or below:
    // END ENQUEUE PARENT ACTION
    add_action('woocommerce_before_checkout_form', 'add_content_above_checkout', 15);
    function add_content_above_checkout () {  echo 'TEST'; }