Hi,
It is normal that the coupon is still generated when you remove that line. That line should just not be there with the new cart system of HikaShop 3, and removing that line was only to fix that error message.
Regarding the fact that the coupon is still generated, that's because you have the virtual coupon turned off. If you turn it off, the system won't create a coupon, it will display as a coupon on the checkout, but it won't be a real coupon, hence the "virtual coupon" name.
Regarding the finish page, I can't say if it's related to the points or not, or why. We would need more information, on how your checkout is configured, how your points plugins are configured, a link to the shop and the steps to reproduce the problem. Also do you have the problem with other payment methods like bank transfer ? Do you have the problem with the default template of Joomla ? Any error message when you do that test ?