Hi Lu Zhang,
It is difficult to identify the exact incompatibility without knowing the specifics of your MPC controller. We are constantly improving and adding to our products, so it is likely that your model built in MATLAB R2017a has features that did not exist in MATLAB R2012b. Therefore, MATLAB R2012b is unable to make sense of these new properties/features, so it will not open the controller object as expected.
I recommend looking at the Release Notes for the Model Predictive Control Toolbox. This will give you insight on what new features in MATLAB R2017a did not exist in MATLAB R2012b, which may help you identify the incompatibility. You can find this information in the following link by adjusting the 'Release Range': https://www.mathworks.com/help/mpc/release-notes.html