Why isn't my Buy X get Y discount code working on my subscription?

[mothertongue]

I've seen this question pop up a few times in the forums, but none of the solutions seem to be working for me...

I have a Buy X, Get Y discount I am trying to make; Customers purchase a pre=paid subscription and get a free coffee mug. 

The Pre-paid Subscription is in a collection called "Subscriptions", which I selected when I was designated which collection I wanted the discount to apply to (see photo).

When I try to check out, I get a " _______  discount code isn't valid for the items in your cart" error message. 

Can anyone figure out why this is happening? I've tried it with different emails, different browsers, cleared my cookies.... No idea how to fix this!

Here is the product page: 

Here is the "Create a Discount" page:

Here is the checkout error message: 

[Blair]

Hey @mothertongue,

Thank you for reaching out to the Community! I'm happy to provide some assistance with setting up Buy X, Get Y discounts, to get this resolved. To provide some context, the Buy X, Get Y discount will require all products to be added to the cart in order to apply as expected. I recommend checking out our Creating Discount Codes documentation for more information on this!

As the customer should receive a free mug if the 6 month subscription-prepaid is purchased, please ensure both items (Buy X, Get Y) are included in the cart at checkout, prior to adding the discount code.

In the case the discount still doesn't work after attempting this troubleshooting step, please Contact Support to allow for further investigation. Please let me know if you have any other concerns. 

[mothertongue]

Thanks for the quick reply-

So I did also try this (added the subscription and also the "free" ceramic mug which is specified in the discount code, and then tried to apply the discount), on two separate browsers and it still did not work I'm afraid.