When I tried to back up the catalog, I couldn't because it's larger than 4 gigs. I have a Macbook pro and the latest version of lightroom. I saw the directions on your website but didn't really understand what to do.
Did the backup actually fail?
The warning message says you will need to download a third-party compression utility to unzip a file bigger than 4 gig.
Catalogs larger than 4GB are now zipped again, but they’re too big for macOS to unzip natively, so you’ll need 7-zip or StuffItExpander if you need to restore a backup.