I recently came across this error message on one of my virtual private servers, where I was trying to have a closer look at the traffic. vnstat is a really cool tool for that sort of thing and on most Linux servers you should be able to install it with
sudo apt-get install vnstat.
I was running
vnstat -l and got the following response:
getting traffic...Error: Unable to get interface "eth0" statistics. Error: Interface "eth0" not available, exiting.