This happens when a model has extermely small time constants or challenging equations to solve. That usually means the model isn't realistic enough yet for the solver to deal with. In your case, the most likely scenario would be a parameterization of the motor or the stepper driver block. Are you able the share the model instead of a screen shot?
Here is a simple example that is close to what you are doing that should work: https://www.mathworks.com/help/sps/ug/stepper-motor-with-control.html
