My Mint 18.X lacks the folder /var/lib/update-notifier/user.d/.
v3.0.24.40626 fails to install with the message:
“/var/lib/dpkg/info/insync.postinst: line 198: /var/lib/update-notifier/user.d/insync-restart-required: No such file or directory”
Creating the folder solves the issue, but this should be handled by the installer.