You've got 60 days... I'd give it a few days for Apple's systems to process the preorders and maybe Monday try adding it to your order, or buying separately.
I'm trying to add AppleCare to my existing order. Using the Apple Store app it shows a separate AppleCare for Apple Watch but when I tried to add to the cart, it says I have to have the watch in cart. How do I add it to my current order?