Hi Minjae Woo,
S-function blocks do not have analytical jacobians which are essential for linearization. That's the reason PID tuner is not able to linearize your model. As a alternate, you can use the frequnecy response estimation technique in PID tuner to get a frequency response of your system out that the PID tuner can use for tuning your gains. Please see below for documentation on:
1) Frequency response estimation with PID tuner with an example:
2)Understand how linearization is carried out in Simulink in Simulink Control Design:
HTH,
Sid