Very few people have read this caveat on the CC's website...
Our system only allows Card Members to pay more than their current balance owed if it has been more than 30 days since their last banking information change or new bank account enrollment. If you enrolled your bank account or changed your banking information recently, this feature will be available to you once the 30-day period has expired from the date you have provided the new bank account information.
...and what they mean by current balance is Statement Balance. Those of us who maximize FICO by "minimizing" credit utilization tend to never/rarely have a Statement Balance, so therefore need to plan way ahead with ACH setting changes FWIW.