This error message means that the Preview cache has got corrupted.
You need to manually go to the location where your catalog is stored and delete the Previews.lrdata file. Do not delete .lrcat file as that is the catalog file.
How do I do that? Once the error message pops up on the opening page and I hit okay the whole program shuts down. Where would my catalog be stored? I don't know how to delete the files
Your catalog is stored on the hard drive.
By default, it is stored in Pictures\Lightroom
You need to browse to Pictures\Lightroom folder through file explorer and delete Previews.lrdata folder.
Thank you so much for your help!
Glad to help.