Hi Serhat,
You might have missed adding your dependencies (multiplier.cpp) to your S-Function in the block parameters. This usually helps Simulink figure out all the CPP files required to do the code generation and avoid any potential linker errors. In my case "multiply_sfunction" is my S-Function and it depends on the CPP file "multiply.cpp"
See the Image below:
From the images I am assuming you are using CPP files instead of tlc to define your S-Function in which case this should work fine.
Refer to the documentation below:
