Install it using:
sudo apt-get install mono-xsp4
This solved the above error.
Optional: get it from Ubuntu software center by searching for
xsp4 server is installed and ready. The project will start running at http://127.0.0.1:8080/
sudo mkdir /etc/mono/registry
and setting the right permissions using
sudo chmod uog+rw /etc/mono/registry
Originally published at xameeramir.github.io.