Vouchers can be created from a reservation, when logging a refund. This article explains how to use a voucher on a reservation.
You can generate a voucher from a reservation, in the payments tab:
A voucher ID will be generated, it can be used as a payment in a future reservation.
You can find the voucher on reservation where it has been created, and on the client file.
On a new reservation, you can choose "client voucher" and add the ID of the voucher already created to log the payment: