We’ve added a new error code, FREE_GIFT_CARD_NOT_ALLOWED, to the SubscriptionBillingAttemptErrorCode enum. This error occurs when a subscription contract includes a gift card product with a purchase price of $0, which is not permitted under Shopify’s business rules.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/new-freegiftcardnotallowed-error-code-for-subscription-billing-attempts