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.