This is because your Pi is running the 4.9 Linux kernel, with which webiopi (a component of the Cayenne Pi agent software) is currently incompatible.
You can switch to the 4.4 kernel with just one command (and a reboot afterwards):
sudo rpi-update 52241088c1da59a359110d39c1875cda56496764
We have an update to the Pi agent in our QA now which should resolve this issue and allow Cayenne to work on both 4.4 and 4.9 in the near future.
Give that command a try (and be sure to reboot the Pi afterward), and
uname -a should show you’re on the 4.4.50 kernel. At that point, the widget should lose it’s ‘unreachable’ label and you should be able to use it OK.