This error means Matlab can’t find the file ‘FAST_SFunc’. If you are running 32-bit MATLAB on Windows, it is trying to find ‘FAST_SFunc.mexw32’; if you’re running 64-bit MATLAB, it’s looking for ‘FAST_SFunc.mexw64’.
You must make sure the file exists and that MATLAB can find it (i.e., the directory where the file exists must be on the MATLAB path). You can look at the CertTest/Simulink_CertTest.m file in FAST v7 for an example of how to add that directory to the MATLAB path.