Payment via Gift Voucher
When adding "Gift Voucher" payments, it depends on the voucher's setup in RetailPro Addon Store Settings what to enter.
If the voucher is set up as "Check for editing" and "Register", then only the voucher's Serial Number is requested:

Make sure to click the "+ Add" button to add the vouchers value as a payment.
If the voucher is set up as "Register" (without "Check for editing"), then both the value and the voucher's Serial Number is requested:

Make sure to click the "+ Add" button to add the entered value as a payment.
If the voucher is not set up as "Register" or "Check for editing", then only the value of the voucher is requested:

Make sure to click the "+ Add" button to add the entered value as a payment.