If you tested it successfully in the sandbox and then in production you are able to make a purchase then the only possibility is that:
1) your app can’t handle two successive calls to updated transactions - the first a ‘failed’ purchase and the second a ‘purchase’ purchase - two because the user needed to update their credit card information
......or......2) you are verifying the receipt at the wrong website
I don't think it is the only reason, because I have the same problem now, when I updated my app to new version - there is number of users calling me with the same problem - in app purchase is not working. I can test it in sandbox only - but everything is OK. The problem is on real accounts.
To be sure - I created new real account in App Store and tried to buy my own application "in real" - it doesn't work. SKErrorUnknown is returned.
What is more stressful in app purchase implementation and store observer is not changed since 2013 !!!!!! I didn't made any changes. I published 20 updates to my app during these 6 years and had no problem. Update no 21 killed me - application works fine - except in app purchase.
Time to die
>there is number of users calling me with the same problem
Time to ping support and see if this is yet another recent IAP probiem on their end...
Not a good sign with xmas and new year coming right up.