In Lightroom CC 2015.8/Lightroom 6.8, you are unable to back up your catalog when exiting the application.
On Windows, Lightroom displays the Confirm dialog with the following 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.
On macOS, Lightroom displays the following erroneous message:
Catalog size is more than 4 GB.
Your catalog size is larger than 4 GB. Please use a 3rd party utility such as 7-zip or StuffItExpander to extract your catalog backup.
Go to Adobe Creative Cloud and download the config.lua file. Copy the downloaded config.lua file into the Lightroom folder mentioned in Step 5.
This config.lua file will reconfigure the old backup behavior in Lightroom.