If you're the Merchant, you need to set yourself up to process refunds in the PayClip app. You can set this up in the BNZ PayClip Merchant portal. To do this, go to the 'Manage Users' tab. Or, if you've been given access to PayClip, check with the person who gave it to you, as they'll be able to set you up to do this.
The refund needs to be processed using the same card from the original transaction. If you're getting a 'Transaction declined' error message, it may be because the customer has not presented the original card for the refund.