This error message signifies that the user attempting to make a purchase already has an existing subscription for the same "product_id". It's important to note that a single "customer_account_id" can only have one active subscription for a given "product_id" at any given time.
To resolve this error, you should first verify the "customer_account_id" of the user and check their active subscriptions. Make sure that you are using the correct "product_id" when making the request, as this is a mandatory parameter for the subscription flow that specifies the subscription product.