This error indicates that the user already has a subscription to the "product_id".

Please note, the user as "customer_account_id" can have only one active subscription within the same subscription "product_id". 

  • Check the "customer_account_id" of the user who is trying to make a purchase and their active subscriptions. 
  • Make sure that you are using the correct "product_id" within your request, as it is an obligatory parameter for the subscription flow that specifies the subscription product.