Message Oracle ORA-27491 - repeat_interval and start_date cannot both be NULL

ERRORrepeat_interval and start_date cannot both be NULL
CAUSEAn attempt was made to set both repeat_interval and start_date to equal NULL for a Scheduler window or schedule.
ACTIONIf either repeat_interval or start_date is set to equal NULL, the other should be set to a non-NULL value.