When you try to join a Zoom activity in Myplace you see Unable to join at this time.

Note: This only applies to scheduled meetings, recurring meetings do not display this message.

If a Zoom activity has been scheduled, the Unable to join at this time message is displayed if there are more than 15 minutes to the start of the meeting.

Approximately 15 minutes before the scheduled start this will turn in to a Join meeting button.

Hosts

If you are the host of the meeting this message is displayed upto 15minutes before the scheduled start of the meeting at which point it will turn in to a Start meeting button.

As the host you can start the meeting earlier via your Zoom client.