show out-of-stock message on the product category

// display an 'Out of Stock' label on archive pages
add_action( 'woocommerce_after_shop_loop_item_title', 'woocommerce_template_loop_stock', 10 );
function woocommerce_template_loop_stock() {
    global $product;
    if (!$product->is_in_stock() )
        echo '<p class="stock out-of-stock">Out of Stock</p>';
}