This is a bad error message (and has been fixed for later releases). The issue here is that the 'serial' function is not supported for code generation because it is a class.
The function 'instrfind' gives a different error message though. It says:
??? The function 'instrfind' is not supported for standalone code generation. See the documentation for coder.extrinsic to learn how you can use this function in simulation.
So the issue here is that you're trying to use functions that are not supported for code generation. As our goal is to generate standalone ANSI-C, there's no ANSI-C equivalent for what you're trying to do (the serial COM layer in MATLAB goes through Java).
Basically, you have two options:
1) Write your own equivalent custom C code to manage these functions and use coder.ceval to call your custom C code. 2) If your plan is to use MEX generated functions (C code itself irrelevant) then you can declare these functions as coder.extrinsic and let the MATLAB environment handle the calls to 'serial' and 'instrfind' at runtime.
