If you're receiving this error when attempting to schedule an event, the cause is scheduling too close to another event.
In settings -> event, you'll see the time set in event automation, if in manual mode.

If it's set to automatic or semi-automatic, the webinar time will go according to the length of the video.

Also, if you have a countdown room, that will also count towards the total webinar time.
* The system adds a few minutes between events as a buffer too, so make sure you plan accordingly.