Woocommerce – changing functions.php in child theme

I am not a coding expert but I want to add some code to change my Woocommerce checkout page and found I needed to add it to my functions.php file. I didn’t have one yet for my child theme so I created it.

I added the code, but it displayed as html under the “Your order” text on the checkout page instead of actually changing anything. I have a screenshot but can’t see how to add it here.

What am I missing? TIA!