Quoting Android developer docs

AlarmManager alarmManager =
    (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
PendingIntent pendingIntent =
    PendingIntent.getService(context, requestId, intent,
                                PendingIntent.FLAG_NO_CREATE);
if (pendingIntent != null && alarmManager != null) {
  alarmManager.cancel(pendingIntent);
}

should be able to retrieve a pendingIntent using it’s requestCode and then cancel it, However, when I try to retrieve it using

AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
Intent intent = new Intent(MainActivity.this, AlarmBroadcastReciever.class);
PendingIntent pendingIntent = PendingIntent.getService(
        MainActivity.this,
        3000,
        intent,
        PendingIntent.FLAG_NO_CREATE | PendingIntent.FLAG_IMMUTABLE
);

The pendingIntent returned comes out to be null even when I have set it using

private void scheduleExactAlarmBroadcast(Context ctx, Class<?> BrdcstRcvr, int requestCode, int afterInMs) {
    AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
    Intent intent = new Intent(ctx, BrdcstRcvr);
    PendingIntent pi = PendingIntent.getBroadcast(ctx, requestCode, intent, PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_MUTABLE);
    if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.S && alarmManager.canScheduleExactAlarms()) {
        alarmManager.setExactAndAllowWhileIdle(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + afterInMs, pi);
    }
}

why is it coming out to be null? Is it not supposed to retrieve pendingIntents using requestCode?

I am trying to cancel the alarm using the requestCode of PendingIntent, however the PendingIntent returned by PendingIntent.getService() is coming out to always be null