By default, add-on fields are only displayed on products that have a price and are purchasable.
However, if you are using your store as a catalogue, or your products are not purchasable, you can add the following line as a snippet to your site:
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|* Display add-on fields on products that do not have a price|
|add_action( 'woocommerce_single_product_summary', 'pewc_product_extra_fields' );|
You’ll also need to follow the steps here to hide the totals field. Just choose ‘Hide totals’ in the ‘Display totals fields’ setting.