The following Shopify discount codes are compatible with subscription orders:
Amount off discounts - These are discounts that give your customers a fixed value or a percentage off of the products in an order, but don't apply to shipping costs.
Free shipping - This is an offer that deducts shipping costs. For example, free shipping on a subscription purchase.
Shopify discount codes can be created in your Shopify admin. Discount codes can be used in addition to subscription group discounts. For more information, please visit our Help Center article here: https://support.boldcommerce.com/hc/en-us/articles/11369207621012-Discount-Codes-for-Subscriptions-for-Shopify-Checkout