Admin product pages custom field displayed in Cart and checkout

Solution 1:

I have tested your code and corrected some portions to make that product custom field appear on cart and checkout pages.

Here is that corrected code:

// Insert a Custom Admin Field
add_action( 'woocommerce_product_options_general_product_data', 'woo_add_custom_general_fields' );
function woo_add_custom_general_fields() {
    echo '<div class="options_group">';

    woocommerce_wp_text_input( array(
        'id'                => 'days_manufacture',
        'label'             => __( 'Days for Manufacture', 'woocommerce' ),
        'placeholder'       => '',
        'description'       => __( 'Insert here', 'woocommerce' ),
        'type'              => 'number',
        'custom_attributes' => array(
            'step'  => 'any',
            'min'   => '1'
        ),
    ) );

    echo '</div>';
}

// Save the field
add_action( 'woocommerce_process_product_meta', 'woo_add_custom_general_fields_save' );
function woo_add_custom_general_fields_save( $post_id ){
$woocommerce_number_field = $_POST['days_manufacture'];
if( !empty( $woocommerce_number_field ) )
    update_post_meta( $post_id, 'days_manufacture', esc_attr( $woocommerce_number_field ) );
}

// Store custom field
add_filter( 'woocommerce_add_cart_item_data', 'save_days_field', 10, 2 );
function save_days_field( $cart_item_data, $product_id ) {
    $special_item = get_post_meta( $product_id , 'days_manufacture',true );
    if(!empty($special_item)) {
        $cart_item_data[ 'days_manufacture' ] = $special_item;

        // below statement make sure every add to cart action as unique line item
        $cart_item_data['unique_key'] = md5( microtime().rand() );
        WC()->session->set( 'days_manufacture', $special_item );
    }
    return $cart_item_data;
}


// Render meta on cart and checkout
add_filter( 'woocommerce_get_item_data', 'rendering_meta_field_on_cart_and_checkout', 10, 2 );

function rendering_meta_field_on_cart_and_checkout( $cart_item_data, $cart_item ) {
    if( isset( $cart_item['days_manufacture'] ) ) {
        $cart_item_data[] = array( "name" => __( "Days", "woocommerce" ), "value" => $cart_item['days_manufacture'] );
    }
    return $cart_item_data;
}

Naturally, this goes in function.php file of your active child theme (or theme) or also in any plugin file.

This code is tested and works.

Reference: WooCommerce : Add custom Metabox to admin order page