In most cases, this is related to the model initialization script. As the error message indicates, you are treating a scalar as a structure.
For example, in the code below, attempting to access "Ts.plant" or "Ts.controller" in the model generate an error message since in the third call the datatype of "Ts" is changed from a struct to a scalar. I would recommend reviewing the model to see if you are inadvertently doing something similar.
Ts.plant = 10e-6;
Ts.controller = 100e-6;
Ts = 100e-6;