Hi,
1. This thread is really old. At that time, PHP 7.1 didn't exist, which indicates that your problem is different from this thread's even though it produces the same effects.
2. This issue comes from the fact that the popup content is displayed by Joomla' com_content extension and in that case, you're normally supposed to provide the id of a menu item of that extension in the URL. But since you're on the checkout, HikaShop only has the id of a menu item of the com_hikashop extension (itself) to work with. So it could try guessing one, or it could add a setting, or you could add that id in the URL yourself with an extra parameter Itemid in order to solve the problem.
3. We actually chose a different way to solve that problem. We've created our own Joomla article displayer URL. That way, HikaShop itself displays the Terms and conditions article and that problem with the SEF system of Joomla doesn't happen. We've added that in the new checkout that is available since HikaShop 3.0.0
So if you still have that problem, it's probably that you're using an old version of HikaShop or that you still have the "checkout legacy" setting activated in the HikaShop configuration.