I experienced a similar error when returning to my site after payment is processed on the PayPal site. (Virtuemart 3.2.14, Joomla 3.9.1, AwoCoupon 3.5.6.9 -- I had not experienced this error with AwoCoupon 2.?)
It turned out that modifying the components/com_virtuemart/helpers/cart.php file to add the following:
<?php
if (!class_exists('CouponHelper')) {
require(VMPATH_SITE . DS . 'helpers' . DS . 'coupon.php');
}
?>
in the emptyCartValues function, in the following context:
<?php
if(!empty($cart->couponCode)){</span><span style="background-color:transparent;color:rgb(0,0,0);font-family:Arial;font-size:11pt;white-space:pre-wrap;">
if (!class_exists('CouponHelper')) {
require(VMPATH_SITE . DS . 'helpers' . DS . 'coupon.php');
}
</span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap;"> CouponHelper::setInUseCoupon($couponCode, true, 1);
}
?>
solved the problem.
I am putting this information here because I found this thread while looking for information on a "class 'CouponHelper' not found" error.