Scope of Article
Refund operation failed
Unable to issue refunds for ACH transactions
This error comes up during the refund transactions through Chargebee and this error is thrown when you do not have sufficient funds in your PayPal balance to cover the amount of the refund or your PayPal account does not have an associated verified bank account.
Here is a document on the same from Paypal.
You can reach out to Paypal support on this and they will be able to help you with this error.