This “meeting is not started” means that you are trying to join a meeting as attendee while no host has started the meeting yet.
A meeting, after being created on zoom, has a status “waiting”. It means it’s waiting for the host of this meeting to start the meeting.
Thus, if no host has already joined and started the meeting, you will get this error when joining as a non host.
Usually you have the options like “participant can join before host”, or “waiting room” on meeting to prevent this error to be displayed depending on the behavior you prefer.