When you create a discount code on Shopify, it automatically defaults to not being able to work for subscriptions.
If you want your Shopify discount code to work on subscriptions, follow these steps:
1. On Shopify, click Discounts
2. Click Create Discount Code
3. Enter your type of discount (example: 10% percent discount)
4. On Purchase Type, click Both so discount code applies to both one time purchases and subscription products OR subscription if you only want it to apply to subscription products.
5. Select your Usage Limits. It defaults to limiting the discount to the first recurring payment.
OR you can apply the Shopify discount code to all future recurring discounts.
6. Review your Summary and if it looks good to you, click Save.