Comment 2 for bug 1876562

This fixed my problem:

sudo apt purge linux-image-4.15.0-99-generic

from:
https://askubuntu.com/questions/1207958/error-24-write-error-cannot-write-compressed-block

Usually if it's a freespace thing, I'll get a popup about /boot not having enough space. That happened during upgrade process, but I wasn't able to purge old space, and it hasn't happened since finishing the upgrade.

There is usually enough space for the current and previous kernel in /boot as well.