I've updated LR and tried the hack given here in December (config.lua), and still get the message
"Lightroom was unable to back up the catalog named "<Your catalog's filename>".
Please check your folder permissions and make sure you have available space on your backup drive and main catalog's drive. "
How do I backup lightroom??? I haven't been able to do it for months, it is an increasing risk to my resources, sanity and sleep.
When the prompt displays for you to backup your catalog, it will indicate where the backup will be created. Take note of that path, and then check that path to make sure it has both read and write permissions. Or, change the path to a new location where you know you will be able to write the backup.