"Meeting Already Underway"

well you could have created another meeting and made someone else an "alternate host". they could have started that meeting but because you created it you can't start another meeting id. that's just a guess though. easiest thing to do is to login to your zoom.us account -->click meetings--> and see if you have 2 scheduled at the same time.  or if you are the admin\owner on your account you should have access to the Dashboard and you can see all meetings running when you are trying to start a meeting.