"createRigidBodyTree" is not a function that ships with RST, or any MATLAB toolbox as far as I can tell. It appears to be part of a file-exchange submission designed to demonstrate some of the robotics-related capability. Did you have a previous version of MATLAB that this worked with, and now that you have installed R2018b it no longer works? If that is the case, you may simply need to add the folders back to the MATLAB search path again.
Alternatively, you can construct a rigid body tree using the normal constructor.