If you are not able to see the Reverse Donation button even after adding it the layout, it's probably because an ExternalId has been populated. You will first need to remove the ExternalId in order to reverse a donation.
  1. Go to the donation record where the Reverse Donation button is missing
  2. Find the ExternalId field and blank it out
  3. Click Save
  4. The Reverse Donation button should now appear