I made the mistake of accepting an upgrade for my Adobe Lightroom. It's been a mess since and I've spent way too much time trying to get it to work. First, it took about 14 hours to build a new catalog. I have now successfully launched the new catalog in Lightroom Classic (was using LR6 prior to that). But I don't have any previews. I tried to "Optimize Catalog" but it seemed to freeze, so I force quit, restarted my computer, and then uninstalled LR because it wouldn't launch. So I uninstalled LRCC, rebooted by computer, and re-installed LRCC. I am now back to where I started - no previews. I am concerned about clicking on "Build Standard-Sized Previews" because that too froze when I tried it, and I'm concerned about optimizing the catalog again.
Am I just not waiting long enough for LR to optimize the catalog? How long does that take? My catalog is only 4.25 GB. LR is on an SSD with 1.5 TB free. I am using a 2009 Mac Pro with 2 x 2.93 GHz 6-Core Intel Xeon processor; 64 GB RAM, an ATI Radeon HD 5770 1024 MB graphics card, and OS 10.13.6.
First, Optimize Catalog will not solve the missing previews issue. How long should optimize catalog take on a 2009 Mac where the catalog is 4.25GB? Probably a very long time, that's a large catalog and a slow (by today's standards) processor, but I don't really know.
You can Build Standard-Size Previews in batches if you want. I would start it going on the whole catalog before you go to bed, and then probably most of it or all of it will be done the next morning.