Hi,
If you don't see the error message even though you activated the "error reporting" AND the "debug mode" settings of the Joomla configuration, then you need to look into the PHP error log of your server for the error message.
Once you have the error message it will be easy to find the source of the issue, but without it, it's hard to say anything. One thing you could try is turn off the "checkout legacy" setting of the HikaShop configuration in order to use the new checkout system. That might help. Also, if you have third party extensions like AWOCoupon, make sure that you have a recent version.