Cannot upgrade to latest version of thunderbird 78

The Ubuntu Desktop Team is currently working on packaging Thunderbird 78. Be patient.

You don't need to add a PPA. The Ubuntu Desktop and Ubuntu Security teams regularly update Firefox and Thunderbird in all supported releases of Ubuntu.

The specific output you received means that nobody uploaded Thunderbird 78 to that PPA.

You can use this ppa which provides the packaged version with no deps like libc6. Thunderbird is actually locked to version 68 on focal, and on 20.10 it is on version 78, but it forces using libc6 2.32.

Any way just use it from here https://www.ubuntuupdates.org/ppa/ubuntu_mozilla_security?dist=focal

sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa 
sudo apt-get update
sudo apt-get install thunderbird

You may also install thunderbird-fr to use French translation for example.