BigCommerce Gift Certificates do not show up as a payment type on the Sell screen if the feature has not been enabled from the settings page.
To fix this, go to your Setup > Add-ons > BigCommerce and you’ll need to first hit ‘Update Permissions’.
This will bring you to the BigCommerce login page. Once you’ve logged in and accepted the updated permissions, you’ll be brought back to the BigCommerce Settings page.
Next, make sure you click ‘Redeem gift certificates in-store’, then hit Save.
Once it’s enabled, the ‘BigCommerce Gift Certificate’ payment type will automatically appear on the Sell screen.