Woocommerce Retrieving custom field value from orders

Some basic issues with code. Since this hook do not have any data supplied to function, you need to get current user logged in.

Secondly, as already mentioned by @antti, get_post_meta accepts first parameter as post id (in this case order id), single value.

function return_s_key_field() {

$orders = wc_get_orders(array(
    'customer_id' => get_current_user_id(),
    'return' => 'ids',
))

$meta_data = array();

foreach ($orders as $order_id) {
  $meta_data[$order_id] = get_post_meta($order_id, 's_key', true);  
}

// var_dump($meta_data);

}
add_action( 'woocommerce_after_available_downloads', 'return_s_key_field' );

Try un-commenting var_dump and then use that data however it is required.