The code you listed;

#include </usr/include/SDL/SDL.h>
#include </usr/include/SDL2/SDL.h>
#include <stdio.h>

Why don't you change it to

#include <SDL2/SDL.h>
#include <stdio.h>

as the first header is where SDL_CreateWindow and other SDL2 functions are declared?

Also you don't need to include both SDL and SDL2 headers. Indeed that could very well be the source of your problem as you would only need to include the version you're using.

If you're following the tutorials from lazyfoo's site, you can check if the ones you're following are using SDL1.2 or SDL2 from their table of contents, as the site actually have the tutorials for both versions.

UPDATE:

I didn't notice that your platform is a Linux platform. Then it is so much easier to solve your problem. The demo that you followed previously was done using SDL-1.2, whereas the gcc error hinted that you're using SDL-2.0, hence SDL_CreateWindow and other undefined errors. You should install SDL-2.0 library and SDL-2.0 development files (which will provide you with the necessary SDL-2.0 headers). You may refer this to SDL-2.0 packages provided by your platform distribution.

As for the compilation, it'll be the same as the tutorial you've followed with a minor change, instead of gcc sdltest.o -lSDL -o sdltest, you'll issue gcc sdltest.o -lSDL2 -o sdltest to indicate that you're linking your code against SDL2 library.

EDIT

A simple SDL program to test your environment. You can use any of the simpler text editor such as nano or gedit or others to edit this, and run the compilation command above to test your setup.

The simplest way to do this is by copy the code, then from your terminal, issue cat > sdltest.cpp and paste the code, then hit [ENTER] and [CTRL-C] to end it. Then you can issue the compilation command as mentioned previously,g++ sdltest.cpp -lSDL2 -o sdltest.

Code;

#include <SDL2/SDL.h>
#include <stdio.h>

int main()
{
    SDL_Window *p;
    SDL_Renderer *w;

    p = SDL_CreateWindow("Game",SDL_WINDOWPOS_UNDEFINED,
        SDL_WINDOWPOS_UNDEFINED,800,640,SDL_WINDOW_SHOWN);
    w = SDL_CreateRenderer(p, -1, 0);

    SDL_RenderClear(w);

    SDL_SetRenderDrawColor(w,255,0,0,255);
    SDL_Rect Rect = {220,140,200,200};

    SDL_RenderFillRect(w,&Rect);

    SDL_RenderPresent(w);

    SDL_Delay(3000);

    SDL_DestroyRenderer(w);
    SDL_DestroyWindow(p);
    SDL_Quit();
    return 0;
}

Hope that helps.