You likely need to install either the xvfb package, or possibly the xvfbwrapper:
First check and see if xvfb is installed:
Xvfb -help
If the command is not found then you can usually install the xvfb package with the following command (or whichever package manager you generally use):
sudo apt-get install xvfb
xvfbwrapper is Python wrapper for running a display inside X virtual framebuffer (Xvfb).
You can install it with:
pip install xvfbwrapper
The error you are getting seems to be a result of either the command or module not being found.