The user who can perform this action must be a Paymaster who has access to all employees.
If there is more than one file to send, the Send button will only be active on the oldest file. Once that file has been Submitted Successfully, the next file will be available to send.
Overnight, the payday file will be processed by IR. The next day, if the status does not update from Submitted Successfully to Accepted Successfully within a day, it is likely the IR have chosen to review the payday files further.