If you have clients bringing in vouchers which were issued prior to you using Simple Salon, you have two ways you can use these vouchers to pay for the client's sale in POS.
'Gift Voucher with No ID' payment method type
For all sales being paid for using a voucher which was issued prior to Simple Salon, simply process the sale through POS using the Gift Voucher With No ID payment method.
Recreate the Gift Voucher via Admin
Alternatively, re-create the Gift Voucher via Admin > Vouchers & Packages, and sell it to the client via POS (this process generates a Gift Voucher ID)—add client to sale, add Gift Voucher to sale, Receive Payment, choose payment type which was used when it was previously sold, Finish Sale, write VOUCHER ID on the voucher.
Then add the client back to POS along with the items they are paying for, Receive Payment, select Gift Voucher as payment type and enter in VOUCHER ID, Finish Sale.
If you do it the second way, you'll need to backdate the first transaction (which sold the voucher to the client). To do this Edit the transaction via the Transaction Report, change the transaction date, and Save. Change the date to the original day when the voucher was originally sold to the client.