I have just downloaded a fresh install from
https://www.opencart.com/index.php?rout ... load_id=47
Set it up on my test server and completed a full cycle from order to checkout.
No Errors.
OpenCart 2.3.0.2
The voucher file is in catalog/model/extension/total/voucher.php
Have you installed any other extensions from your fresh install ?
Maybe a vqmod or ocmod extension or an extension that requires any of these functions. And have you downloaded the compatible versions ?
Have you tried to disable all the other "extra" extensions from the system and clear the cache each time to determine what extension is maybe causing the error ?
Hey and if all else fails......
Do a fresh installation.