In Lightroom CC 2015.8/Lightroom 6.8, you are unable to back up your catalog when exiting the application.
Lightroom CC 2015.8/Lightroom 6.8
Windows and macOS
To resolve these issues on Windows and macOS, follow the workaround steps mentioned below:
(Win) Choose Edit menu > Preferences.
(Mac) Choose Lightroom > Preferences.
When the Preferences dialog appears, select the Presets tab.
In the Presets tab, click the Show Lightroom Presets Folder button. Lightroom opens the root preset folder in the File Explorer (Win)/Finder (Mac).
In the File Explorer (Win)/Finder (Mac), navigate to the Lightroom folder.
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.
You should be able to back up your catalog now.