A =

To be explicit:

ss2tf() does not work on symbolic variables. Very few of the functions in the Control System Toolbox will work with symbolic variables (the ones that do work are more or less by accident.)

See https://www.mathworks.com/matlabcentral/answers/2144394-using-ss2tf-when-your-matrices-are-variables-without-values-yet#comment_3234629 for how it would have to be done symbolically.