Hi there, civib.
I'm more than willing to help you get past this error so you can access your company file again.
We have to make sure to close your QuickBooks Desktop for Mac entirely and launch it again to see if it works. You can follow JessT's steps by going to the Force Quit Applications window.
Also, I'd suggest reaching out to our Technical Support Team. That way, they'll be able to investigate this further and see what's causing this error.
Keep on posting here if you need anything else. We'll respond as soon as we can.
