Make sure that WebIOPi is running. At your Raspberry Pi’s console, type (or copy and paste):
$ ps aux | grep webiopi
You should see output similar to this:
root 6254 0.0 2.6 33892 11860 ? Sl Oct29 11:01 /usr/bin/python3 -m webiopi -l /var/log/webiopi -c /etc/webiopi/config
If you don’t, the WebIOPi is not running. You may wish to install WebIOPi to run automatically as a daemon every time your Pi restarts (which is not the default).
$ sudo /etc/init.d/webiopi start
Once you have confirmed that WebIOPi is running, try again.