I just started having the same issue today. I've reset Origin, deleted EVERYTHING origin from my mac, installed a new one, even went into time machine to load one from a few days ago and still get the same message saying I can't open it because someone else is using it. No, it is not open on any other computer, this is it. I can't get in no matter what I do.
February 2020 - last edited February 2020
This also happened to me, I was on zoom and sims 4 was open in the background, so I thought that when my computer did a weird restart without me restarting it my computer had crashed from the two applications. I opened it back up, and my zoom was working but my sims 4 was not there. Later that night I went to play sims and it showed the message, 'You can't open the application "Origin" because someone else is using it.' with a finder icon next to it. I followed the advice of #2 by going to the go and library folders and downloading a new origin, but it did not work. I followed the advice of simply resetting my computer, and that did work. Hope this helps
I had the same problem, and I was going to reinstall the game but decided to restart my Mac first, and then Origin just opened like usual when I log in, so that might work for you guys too?