Error 12 happens on many many purchases....lose of sales, must fix, please help!

I am still receiving so many orders and they are getting failed due to “error 12”. I purchased (neighborhood theme). My developer went to see what was wrong but could not find anything. It’s just too many people that are trying to buy but cant. I also tried our paypal merchant and they did not have anything either. This is still a major problem. One customer tried 3 different cards with no luck! Please help! any ideas would be appreciated on what to check!

Have you checked

That error is usually not related to the theme and more the transaction