If the transaction has not posted ie still pending, you will not be able pay.
I thought same thing but OP says statement cut with a due date?
I had this happend earlier this month. I couldn't pay the "pending" charge until it cleared. The charge was also made the day the statement cut but posted then next day. Account balance was $0, but avail balance included the "pending" charge. My CR reported it as a $0 because the "pending" charge didn't clear. I wonder if this is the case with the OP...