Simulink will pass the current folder as well as any specified include directories path(s) to the build process. If the header file is in neither of these locations, the error above will be thrown by the compiler. Note that the MATLAB search path has no impact on the directories included in the build process.
Check to make sure that the path to the header file is defined in this location:
Simulink Model Settings > Code Generation > Custom Code > Include Directories
Refer to the link below for things to consider when adding paths to the include directories list:
If the header file is related to an S-function, a more advanced, but also more elegant approach to manage your S-function dependencies would be to write a "makecfg.m" file. Through this file, you can specify all dependencies and includes for the S-function. This is particularly recommended if you aim to distribute the S-functions to other users:
