There are probably a lot of reasons for this to happen but I was getting this error the other week when buying multiple tickets and applying pay with miles. My issue was the purchase of the second ticket was allowing me to apply more miles than I actually had. The system that was calculating how many miles I had available when ordering the ticket was out of sync with my actual current mile balance. In my case I could manually select less miles to apply and then it went through. But if the system is out of sync and you actually have the miles you may just have to call if you are also trying to "Pay" with miles.