Please ensure that the Customer has a valid gift card on file, see How do I view/edit my Customers' Gift Cards?
Gift Card Payments can also be added to Sales in the Checkout screen, see How do I checkout a Reservation?
Tap the green + icon > Add Payment > Gift Card
A list of Gift Cards assigned to this Customer will be displayed:
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