Hi,
Yes, the donation was working properly in the past and the "warning" you shown is just a warning and do not have any special link with the donation plugin.
So without any more details about your configuration, how you have set-up the plugin and the rest ; it will be difficult to give you any useful answer.
But, because HikaShop 2.6.0 is a little bit old and that we fixed and improved of lot of elements since September 2015, still want to recommend you to perform an update. First to get the full Joomla 3.6.5 support !
www.hikashop.com/support/documentation/5...ashop-changelog.html
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.