Can shoppers apply a CashBack gift card to an existing subscription?
When shoppers select store credit as their CashBack option, gift cards are created via Shopify API. One of the limitations of this is Shopify does not allow gift cards to be used for subsequent subscription payments.
As a workaround, you can credit the shopper the value of the gift card for an upcoming subscription or refund the shopper for a past subscription, then suspend the CashBack.