Depending on when the donation was given, it's quite possible you don't yet have the ability to refund the transaction. (The Refund Transaction button will be faded out.) This is simply because you can't refund money you don't have, and you can't cancel a transaction in the middle of it being processed. As soon as the donation has been cleared and is deposited in your bank account, you'll be able to refund the donation.
Depending on how the donation was given (either via credit card or ACH) will determine how long until you'll be able to refund the transaction. To learn more about gift payout schedules, click here.
In summary, all gifts are grouped together in "batches." The cutoff time for batches is at 8:45pm EST. Unless there is a specific issue with a credit card transaction, all credit cards donations should be deposited the next day. All gifts donated via ACH will take between 7-14 days. Click here to learn more about ACH transactions.
For information on how to process a refund once you have the ability to click the Refund Transaction button (i.e. once the transaction has been cleared and deposited into your bank account), click here.