Using functions.php for outputting ACF

Hi,

I know that this code will work if e edit the child theme or us a code plugin.

add_action( 'woocommerce_single_product_summary', 'product_subtitle', 5 );
  
function product_subtitle() { ?>

<?php if(get_field('my_subtitle')) { ?>
	<div class="style_my_subtitle"><?php the_field('my_subtitle'); ?></div>
<?php }
}

However I would like to use the functions.php instead and wrap the ACF output into a div for styling. If I use the code above in the functions.php i get a critical error. Can you please point me into the correct direction which code I have to use to use a div in the functions.php

Kind regards,
Mike