Hi,
Did you modified a file of the HikaShop core ?
The HikaShop product class (administrator/components/com_hikashop/classes/product.php) contains the function "addAlias" so the error should exists.
Please check that your class product file has not been modified and that it has the right version.
If you still have some problem, you can download and install HikaShop 2.1.0 in order to update all of your files.
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.