您的代码中存在一些错误和缺失，请尝试以下操作：

add_action( 'woocommerce_before_add_to_cart_quantity', 'display_product_total_amount_html' );
function display_product_total_amount_html() {
    global $product;

    $meters_per_qty  = 0.5;
    $display_price   = wc_get_price_to_display($product);
    $formatted_price = '<span class="price-amount">'. number_format($display_price, 2) . '</span>';

    // Output product total price amount
    $price_string = sprintf( __('Add %s for %s', 'woocommerce'),  
        '<span class="qty-meters">' . $meters_per_qty . ' meter</span>',
        str_replace('0.00', $formatted_price, wc_price(0)) );

    // Output product total price amount
    echo '<div class="total-price_qty" style="margin-bottom:20px;">'.$price_string.'</div>'
    ?>
    <script>
    jQuery(function($){
        $('input[name=quantity]').on( 'input change', function(){
            const qty = $(this).val();
            if ( qty != 0 ) {
                const lengthTxt = qty > 2 ? ' meters' : ' meter';
                $('.total-price_qty .price-amount').html( parseFloat(<?php echo $display_price; ?> * qty).toFixed(2) );
                $('.total-price_qty .qty-meters').html( parseFloat(<?php echo $meters_per_qty; ?> * qty)+lengthTxt );
            }
        });
    });
    </script>
    <?php 
}

代码位于子主题的 functions.php 文件中（或插件中）。已测试并有效。