I am using unirgy_giftcert gift certification plugin. When I use gift code during checkout, I get following error message Can't find implementation of payment method Unicert
Order seems to be completed, because, when I go to My Cart page, cart is blank. Now When I try to complete the order in back end, I get the same error.
Ok, I've found answer of my question, after doing some R&D with my site. Note : If you are using unirgy_giftcert and Subscription plugin, please note, you can not buy subscription products using Gift certificate codes Actually, you can buy (but you must not), but, can not make full payment using gift certificate, because, it might create some problem in future devilries. My site is a subscription based product selling company, that is why I was getting error.
I hope this will help someone.