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 are added to a Sale in the in the PAYMENTS section
Tap + Add Payment, enter the required amount (if needed), tap Gift Card
A list of Gift Cards assigned to this Customer will appear
Alternatively, you can use the Search tool to search all the Gift Cards in your system by either the Customer name or the Gift Card number
Tap on the relevant Gift Card to use it
The Gift Card Payment will appear in the PAYMENT section
Alternatively, you may choose to use the Gift Card shortcut on the right hand side of the screen
Proceed with adding any other required actions, then close the Sale