Welcome to the community @mea 

To answer your specific questions:

Is the coupon a Unique code? and is it being generated? Is there somewhere in Shopify that I should triple-check everything is running correctly?

Yes. A dynamic coupon creates a unique code. From what you’ve shared it looks like it’s being generated OK. You can check this in Shopify in the ‘Discounts’ tab, You should see instances of a discount called ‘HolidaySubscriber2024’. Click on that and then ‘View all codes’ (top right) to see the list of codes generated. You can also do a live test by sending a campaign that has the HolidaySubscriber2024 tag in the body to your test email.

Could I have use an easier coupon code like Holiday10 (if this is a single use coupon)? 

Yes you can use a static code. Considerations to be aware of are that it can be shared, and the expiry date is the same for everyone.  

As to the skipping, you need to click on ‘skipped’ to see the reasons. You do have smart send enabled on your flow email so that may be the issue. If any profiles entering that flow after subscribing have received any other emails with the default smart-send window of 16 hours, they’ll be skipped. For a sign-up flow like this, I recommend disabling smart send on that first email.

Hope that helps

Andy