Zoom Meeting

This message is commonly caused when the host is in fact hosting a different meeting at the time your associate tried to join a meeting.

Assuming that you are in the meeting that you're are hosting, waiting for your associate to arrive, I would then click on the Green Shield on the top-left of your Zoom window and click on Copy Link and send it to them.

Somehow or another they are trying to join a meeting that you are not currently hosting.