Please ensure that the Customer has a valid gift card on file, see How do I sell a Gift Card?
Payments can also be added to Sales in the Checkout screen, see How do I checkout a Reservation?
Payments in a Sale are found in the Payment section
Click Add Payment to add a new Gift Card Payment
Enter the required amount and click Gift Card
A list of Gift Cards assigned to this Customer will appear, click Pay against the required gift card
Alternatively, it is possible to use the Search field to search for and pay with a different Customer's Gift Card, or a Gift Card in the system which is unassigned to a Customer
Click Close Sale to close the Sale or
Continue Editing to return to the open Sale