@GolamMostafa I think a lot of the other posters might have covered everything I would have replied here, but we are essentially saying the same thing. In fact, @alto777 was the first person to directly answer the OPs question. It's not an Arduino only thing so it has nothing to do with setup or loop.
The precomplier must see a function declaration before it sees a function call. The function can be both declared and defined before the call, but at minimum, it must be declared.
If you are active on this forum and copying and pasting, and asking these questions, you are a programmer. The concept of declaration and definition are a bit deeper than where you may be at right now, and how fast you progress in programming is your own choice. Words like constructor, destructor etc. may not be clear to you now, but the long story made short is that you could replace the declaration line where it is with the actual function from below and have it work. The function would be declared and defined in advance of the call.