为什么我的百分比代码的woocommerce优惠券不起作用？

Question

I actually want a percentage coupon system with the Maximum discount of xxx $.

This is the code that i am using for the WooCommerce plugin within WordPress but it's not working properly.

add_filter( 'woocommerce_coupon_get_discount_amount', 'woocommerce_coupon_get_discount_amount', 20, 5 );
function woocommerce_coupon_get_discount_amount( $discount, $discounting_amount, $cart_item, $single, $coupon ) {

    $max_discount = get_post_meta( $coupon->get_id(), '_max_discount', true );

    if ( is_numeric( $max_discount ) && ( $max_discount > 0 ) ..... && ! is_null( $cart_item ) && WC()->cart->subtotal_ex_tax ) {

        $cart_item_qty = is_null( $cart_item ) ? 1 : $cart_item['quantity'];
        if ( wc_prices_include_tax() ) {
            $discount_percent = ( wc_get_price_including_tax( $cart_item['data'] ) * $cart_item_qty ) / WC()->cart->subtotal;
        } else {
            $discount_percent = ( wc_get_price_excluding_tax( $cart_item['data'] ) * $cart_item_qty ) / WC()->cart->subtotal_ex_tax;
        }
        $_discount = ( $max_discount * $discount_percent ) / $cart_item_qty;

        $discount = min( $_discount, $discount );
    }

    return $discount;
}

Can you please help me out to fix it?