There is a discrepancy between the Task Coach website and availability for Ubuntu 20.04.
Ubuntu versions supported
Ubuntu 10.04 LTS ("Lucid Lynx") and newer.
Prerequisites
Python 2.7 and wxPython 3.0.0.0-unicode (or newer).
Installation
Add ppa:taskcoach-developers/ppa to your Software Sources..
However, attempting to add the PPA fails on Ubuntu 20.04.
Cannot add PPA: ''This PPA does not support focal'
We don't know if we'll keep with PPAs, development is near-stalled. Maybe AppImage or something will work.
In the mean time, there are some annoying workarounds to get Task Coach installed. See this discussion: https://answers.launchpad.net/taskcoach/+question/690591