How do I echo Sku?

I’m trying to echo sku on a custom pdf packing slip. I’m using this code:

<?php $items = $orderData->get_items(); if( sizeof( $items ) > 0 ) : foreach( $items as $item_id => $item ) :
                 if(get_post_meta($item['product_id'],'_warehouse',true)==$VendorID):
                    $_product     = apply_filters( 'woocommerce_order_item_product', $orderData->get_product_from_item( $item ), $item );
                    $item_meta    = new WC_Order_Item_Meta( $item, $_product );
                ?>

with this piece of code not working
<?php echo $item['sku']; ?>

but this one does
<?php echo $item['product_id']; ?>

Can someone please explain to me in layman’s terms what I am doing wrong here? I’m not very knowledgeable in PHP so any placement help would be much appreciated as well. Thanks in advance!

https://wordpress.org/plugins/woocommerce/