Cannot make a change to a subscription that generates a credit for an uncompleted transaction

You can't make a change to a subscription that generates a credit for an uncompleted transaction

Error Message	`The subscription update you're trying to make generates a credit for an uncompleted transaction.`
Error Code	`subscription_credit_creation_against_uncompleted_transaction_not_allowed`
Status Code	`400`

Common causes

You're trying to make a change to a subscription that generates a credit for an uncompleted transaction. This would mean crediting an amount that hasn't yet been paid, which isn't permitted.

Collect payment for any uncompleted transactions, then try the update again.
If you don't need to create a credit for this change, send your request with do_not_bill as the proration billing mode.