Hi,
The fact that you get "registration is not allowed" is because you have turned off the new user account creation setting in the options of the Joomla user manager. Please turn it on.
And if you want to activate the guest checkout of HikaShop, you can set the "registration" setting of HikaShop in the Checkout tab of the HikaShop configuration to "guest".
Finally, if you want both, you want to change the "Display method for registration" setting there to "switcher" and select both "guest" and a registration choice in the "registration" setting of the HikaShop Checkout tab configuration.
And yes, you can't use an email address to guest checkout if that email address is already used for a user account. In that case, instead of guest checking, you can simply login via the login form (that you'll see when you turn back on the "login" setting of the HikaShop configuration). And if you want to try the guest checkout with a real email address but don't want to create a new gmail account or something like that, you can use an email address from there
www.throwawaymail.com/