It appears that oyu have a 3rd party toolbox installed. The 'tf' function is usually part of the control system toolbox and it can handle what you have posted just fine. The error is pointing to a different tf function in the DynamicSystem (non MathWorks) toolbox.
I would suggest doing a
which -all tf
and seeing what tf functions are available. If there are others available you should use the MATLAB ones instead.
Your R2015b Student Version may or may not have the Control System Toobox (which is a prerequisite for "tf" transfer functions. It really depends on what you have purchased. Can you provide the output of the "ver" command? It returns allinstalled toolboxes.
What puzzles me is the "SR_Control_Toolbox" name. To my best knowledge, this links to a product that is pretty old and not available for R2015b. Can you also share the return result for the command
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