I found the error!

The error wasn't because of shader, it was in main.cpp file..

The error was this:

    Shader shader("shaders/shader.vert", "shaders/shader.vert");

It was passed only vertex shader, and actually yes..The second shader was compiled as frag shader..

I fixed it:

    Shader shader("shaders/shader.vert", "shaders/shader.frag");

Thank you!