This error means that some kind of change is required by IATS. In this case, since you are getting the error message while attempting to refund an ACH transaction through IATS, it's probably because the account doesn't have ACH refunds enabled.

The owner of the account must email iatscs@iatspayments.com and include the client code and request that ACH refunds be permanently enabled. The owner of the account might be Luminate support in some situations.