julianweiland on "[Plugin: WooCommerce - excelling eCommerce] Header Code to order completed page"

I want to add a "Facebook convention pixel" to the order completed page.
I tried this:

if(isset($order)){
	if ( $order->has_status( 'completed' ) ){
		echo "<!-- Code for Facebook Conversion Pixel -->
	};
};

but when i do this I can't add a item to the cart. I always get the message it has been added to the cart. But at the cart Page I get the message Cart is empty. The Cart sidebar widget is not beeing displayed to. Why does this happen?

I just want to add header code to order completed Page.

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