Ok, without knowing exactly how you setup the coupon its impossible to tell you what the problem is. There are 2 things you can try.
1) go into admin screen, awocoupon and setup an absolute generic coupon. So only fill in the coupon code and the value field. Then try it, that should work for anybody, whether logged in or not.
2) if you are a programmer and know your way around the code, open this file: www/administrator/components/com_awocoupon/assets/virtuemart/ps_coupon_process.php. In the function return_false($message) change
$GLOBALS['coupon_error'] = $VM_LANG->_('PHPSHOP_COUPON_CODE_INVALID');
$GLOBALS['coupon_error'] = $message;
This will display the exact error message you are receiving when entering an invalid code, instead of the generic.