Best answer by Eva_Boost
You'd get error D6 if you've used the wrong key to top up, D2 would also mean the same thing.
We'll need to get a new key sent out to you, or give you a code to pick up a new key from a PayPoint in your area.
Just send us a message on Facebook or Twitter, you'll need to confirm your name, DoB and account number. If you don't use Facebook or Twitter, just give the team a call on 0330 102 7517 - they're available Monday-Friday 8am-6pm and Saturday 9am-1pm.
*Update - 11/07/2019 - Added links to Facebook and Twitter accounts*