When you install the plugin, you can create vouchers and distribute to your users. However, this default setting determines of your vouchers should be available for logged in users (registered users) or all your app users.
1. Admin > Settings > Vouchers. Under the Generaltab, enable the 'Not login available' button available to unauthorized users.
When this field is not enabled, users who use your app as guests, won't see the button, 'Apply Voucher.'