Reply To: add a product on the cart to get discount on other category

[Soft79]

Probably because it’s a variable product. Try this:

add_filter( 'woocommerce_coupon_get_discount_amount', function( $discount, $discounting_amount, $cart_item, $single, $coupon ) {
  $COUPON_CODE = 'COUPON_CODE'; // replace with the coupon code
  $DISCOUNTED_CATEGORY = 123; // replace with the category that is discounted

  if ( 0 == strcasecmp( $coupon->code, $COUPON_CODE ) ) {
    $_product = $cart_item['data'];
    if ( $_product->get_parent_id() ) {
      $_product = wc_get_product( $_product->get_parent_id() );
    }
    $cat_ids = $_product->get_category_ids();
    if ( ! in_array( $DISCOUNTED_CATEGORY, $cat_ids ) ) {
      $discount = 0;
    }
  }
  return $discount;
}, 10, 5 );