Dear Taha,
To compare the nonlinear response with the linear response, the key thing to keep in mind is that the linear model is defined in terms of perturbations about the operating point e.g. y = y_op + dy for system outputs, so, you should either add the operating point value (y_op) to the output of the linear model or subtract the operating point value from the output of the nonlinear for comparison. See our TORQUE paper on the FAST v8 linearization process for more information: nrel.gov/docs/fy17osti/67015.pdf.
Recently another user on our forum posted some results of their comparison between the linear and nonlinear solution–you can use that topic for additional guidance: PROBLEMS TO VALIDATE LINEARIZATION WITH TORQUE CONTROL.
I’m sorry, but I’m not familiar with that error. Do you have a FAST primary input file named, “FAST_Test18.fst”?
Best regards,