In the link it has been mentioned that only Ubuntu Vivid and Wily are officially supported. Is that the reason why I get the above output ?
For the repository that you linked to,
qtox-unity is indeed not available for Ubuntu 16.04. However, you can still install the qTox client for Tox following the instructions at https://github.com/tux3/qTox and https://software.opensuse.org/download.html?project=home%3Aantonbatenev%3Atox&package=qtox:
sudo sh -c "echo 'deb http://download.opensuse.org/repositories/home:/antonbatenev:/tox/xUbuntu_16.04/ /' >> /etc/apt/sources.list.d/qtox.list" sudo apt-get update sudo apt-get install qtox
If you want, you can also add the repository key to APT:
wget http://download.opensuse.org/repositories/home:antonbatenev:tox/xUbuntu_16.04/Release.key sudo apt-key add - < Release.key