I saw that there were some new updates to MacOS Catalina, so I tried to install them through System Preferences / Software update:
After trying to get the update to install (forcing a restart), I noticed that it would not install — even after multiple tried with full battery and plugged into power.
After a while I assumed I didn't have enough free space, and tried again — still no luck.
I found somewhere a command to install the updates from the terminal, so I tried it:
sudo softareupdate -i -a
Then the truth was revealed:
There is nowhere in the Software Update prefpane that states how much space is required or what is blocking the update. This should be a relatively simple UX update that will allow less technical users to be safer.