Ah dang, in that case I would have said the guide was incorrect and you should contact Shopify support, but since you’re now talking about packing slips you’ll need to consult a different guide.
I found this: https://help.shopify.com/en/manual/shipping/setting-up-and-managing-your-shipping/packing-slips-variable-list
Tip: Shopify has guides for almost everything in the store. Whenever you need help just search “shopify [what you’re looking for] guide”. The guides are pretty good, they can help you understand what you’re editing and make you a stronger store manager. And of course post in the forums if you need help too, plenty of us like to help.
I read that guide and don’t see order attributes available, so you may have to change your flow and use either order notes or line item properties (you can see how to access both of those variables in the guide).
Example:
{{ order.note }}
If you’re not sure how to change the attributes to notes, I would contact either the app or the developer that set that up, they might be able to do it.