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I am not able to delete some of the subscriptions that I created. Why is this happening?

Summary

You will not be able to delete a subscription if the subscription's invoice amount is adjusted with another subscription's payment under the same customer record. Refer to this document for more details.

Info: 

1. An Adjustment Credit Note is used to adjust the amount of an already existing invoice that is in Payment Due or Not Paid state. This type of Credit Note can be created when the price of an invoice has to be reduced as a discount to the customer.
2. The Adjustment Credits appear as "Adjustments" in an invoice.

An Adjustment Credit Note is automatically created when

• An invoice is written off

• A subscription is changed with “proration” enabled and the current term's invoice is in the Payment Due or Not Paid state

To remove the Adjustment credit note, 

1. Go to the invoice 

2. Remove the credit note applied as shown below,
   

Ensure none of the invoices was paid using another subscription’s credit note before you delete the particular subscription.

Warning:

1. The delete action carried out is irreversible: all the data related to the Subscription, such as Transactions and Invoices, will be deleted.
2. Only Admins are allowed to delete a Subscription.
3. Deleting a subscription will cause all the linked invoices and transactions to be deleted

Related articles and Documentation

What happens when I delete a plan in Chargebee?

My customers cannot add addons to their existing subscription