First, I had to check which it was. In one place, you wrote circularCavity. In another, you wrote cavityCircular. Google is my friend though. :)
It tells me that something called cavityCircular does exist, here is the doc:
https://www.mathworks.com/help/antenna/ref/cavitycircular.html
Sadly for you, it seems to have been introduced in R2017b.
So you cannot use it in R2016a. Code from new releases cannot be used by old releases. You would need to update your MATLAB release to get that toolbox. Or you would need to write it yourself.
