Any changes you make to PayAll setups, whether that be new ones or cancellations, take 24hrs/overnight to be reconciled back to the Citibank computers to take effect. It is not instantaneous like you imagine it to be given that it all takes place on a phone which is logged in to Citibank. The front end user experience is 21st Century, but the back end processing is from the Victorian era circa-1850's. When you cancel a payment, it is only cancelled in the 21st Century - which doesn't count. You have to wait for it to also be cancelled manually by a Victorian-era banker wearing a monacle, sitting at a desk with a green banker's lamp, updating the ledger with a fountain pen. This only happens in the dead of night.
It is either a mark of just how inefficient Citibank's systems are that they can't cope with anything taking place more often than once in any 24hr period, or else it is an intentional anti-flood mitigation measure to prevent people from repeatedly making and cancelling and then remaking new PayAll payment requests in a very short period of time. Either way, the moral of the story is, work out exactly what you want to do offline first using paper+pen if you have to, before you pick up the phone and start trying to do it live in the app, because you can't afford to make a mistake unless you want to wait 24hrs between each one before you can fix it.