Hello,
Indeed there is no such message in the plugin or in the HikaShop core.
But the paypal advanced plugin do not display an error message just like that, there should be other text "around" which would indicates more about the context and from where the error is coming from (what kind of API call, etc).
So it would be interesting to have the exact error message and maybe a screenshot of the page with the message.
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.