Older Ubuntu releases (Ubuntu 12.10 and 12.04)
To enable installation on older Ubuntu releases such as Ubuntu 12.04 and Ubuntu 12.10 (and their derivatives), you will need to add a second repository to your system, in addition to the generic Debian/Ubuntu repository (if you don’t use sudo, be sure to switch to root):
echo "deb http://download.mono-project.com/repo/debian wheezy-libtiff-compat main" | sudo tee -a /etc/apt/sources.list.d/mono-xamarin.list && sudo apt-get update
После чего попробуй продолжить установку.