Hi,
When it says that it means that the ACARS didn't record the moment of the "Takeoff" status or the "Landing" status.
Also, it could had happened because of 2 main reasons:
1. You turned it on when you already got airborne.
2. For some reason there was no internet communication between your computer and EHM site on that moment.
Sorry if it was second, but, as you know, we don't control that and it could happen to anyone

Regards,