Background I've installed Ubuntu 18.04 on my new laptop.
Problem The /var partition was too small. The system complained that only 200 MB was left.
Solution Rebooted with my live USB. Opened GParted. Moved /tmp partition to the left and grew it to 6 GB. Grew /var partition to 16 GB Click Results Details [TL;DR] Here's the GParted generated log.
GParted Details GParted 0.30.0 --enable-libparted-dmraid --enable-online-resize