Sorry that Lightroom Classic CC 7.3.1 is crashing when you attempt to backup a catalog.
Are you on Windows OS or MAC OS, this is a known issue on MAC OS.
When backing up your catalogs on MacOS, Lightroom Classic doesn't compress (zip) catalogs that have a file size less than 4 GB. As a workaround to this issue, manually compress the backed up catalog files. Compressed files take up less hard disk space. By default, Lightroom Classic saves backed up catalogs to the following location on macOS:
/Users/[user name]/Pictures/Lightroom/[catalog name]/Backups
As you've mentioned that the app is crashing, could you please provide us the crash report so that we may diagnose it? Refer Submit a crash report for Adobe Photoshop and Lightroom