'The catalog could not be opened due to an unexpected error.'
I'm on a Mac running High Sierra 10.13.6.
I had trouble opening my catalog last week and upgraded lightroom and was able to open the catalog.
Today I tried to open and got the same error again.
Uninstalled and reinstalled but still can't open it.
My catalog lives on an external drive. Seems like it might have something to do with this.
I tried to create a new catalog to test and it'll only allow me to do it on the desktop itself.
The problem appears to be with your drive based on the fact that you can create and open a catalog on your desktop. Do you have another drive that you can test with? If you can create and open a catalog on another drive, move the problem catalog there and see if it opens on that drive.
Do you have a backup of the problem catalog?