If you are seeing a "Payment Response Desc" of "Missing Masked Credit Card Number", then please check the related Merchant Facility in Salesforce and set the Credit Card masking to "4X....4X". This will now work for transactions created from now on. For transaction created prior to this date, you'll need to refund through the Authorize.net portal.
If you are seeing a "Payment Response Desk" of "A valid referenced transaction ID is required.", then check the original Payment Txn you are trying to refund. If the "TxnRef" has a value of "0", then this indicates you do NOT have Authorize.net in Live Mode. This is required, even for Sandbox.
If the original transaction to be refunded has not yet been settled (generally 24 hours form the payment), then the "Void" checkbox needs to be selected and the refund must be for the full amount (i.e. not partial).