Hi,
If in your Joomla global configuration you did not allowed the registration, HikaShop won't be able to create any Joomla user account, because the registration is forbidden for Joomla and all of the components.
That's why HikaShop check that it has the right or not to create Joomla accounts and display this message, if it knows that it won't be possible.
So please, check your Joomla configuration like Nicolas asked you.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.