A customer wants to pay with a gift voucher, bought before you worked with the numbering in Optios? Then you simply have to change the setting again to be able to enter it.
Go to:
Settings
Register
Uncheck "Giftcard tracking active"
Save.
You can now register the gift voucher (without numbering)
❗ After this payment you must check it back to give the next gift voucher a number again. Once all the "older" gift vouchers have been returned, you no longer need to adjust the setting.