The developer of the module has kindly helped me out with a similar fix. He also mentioned that PS 8.x is up to today still including swiftmailer.
If anyone else is using awocoupon and is facing the same issue, here's the solution:
From the (old) TB 1.4 zip, copy the folder
/vendor/swiftmailer/swiftmailer
to
/modules/awocoupon/asset/swiftmailer
So this file should exists:
/modules/awocoupon/asset/swiftmailer/lib/swift_required.php
Then in the file
/modules/awocoupon/helper/mail.php on line 12 is this:
<?php
public static function sendMail($from, $from_name, $to, $subject, $body, $mode = 0, $bcc = null, $attachments = null, $is_embed = false, $string_attachments = null, $cc = null ) {
?>
Right after that add this:
<?php
include_once _PS_MODULE_DIR_.'awocoupon/asset/swiftmailer/lib/swift_required.php';
return self::sendMail3($from, $from_name, $to, $subject, $body, $mode, $bcc, $attachments, $is_embed, $string_attachments);
?>
hope others find this useful.