Something in shader is messing the rendering function

    #include <GL\glew.h>
#include <GLFW\glfw3.h>
#include <iostream>

// GLM library 
#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtc/type_ptr.hpp>

using namespace std;

int width, height;

// Draw Primitive(s)
void draw() {
    GLenum mode = GL_TRIANGLES;

    GLsizei indices = 6;

    glDrawElements(mode, indices, GL_UNSIGNED_BYTE, nullptr);
}

// Create and Compile Shaders
static GLuint CompileShader(const string& source, GLuint shaderType) {

    // Create Shader Object
    GLuint shaderID = glCreateShader(shaderType);
    const char* src = source.c_str();

    // Attach source code to Shader object
    glShaderSource(shaderID, 1, &src, nullptr);

    // Compile Shader
    glCompileShader(shaderID);

    // Return ID of Compiled shader
    return shaderID;
}

// Create Program Object
static GLuint CreateShaderProgram(const string& vertexShader, const string& fragmentShader) {

    // Compile vertex shader
    GLuint vertexShaderComp = CompileShader(vertexShader, GL_VERTEX_SHADER);

    // Compile fragment shader
    GLuint fragmentShaderComp = CompileShader(fragmentShader, GL_FRAGMENT_SHADER);

    // Create program object
    GLuint shaderProgram = glCreateProgram();

    // Attch vertex and fragment shaders to program object
    glAttachShader(shaderProgram, vertexShaderComp);
    glAttachShader(shaderProgram, fragmentShaderComp);

    // Link shaders to create executable
    glLinkProgram(shaderProgram);

    // Delete compiled vertex and fragment shaders
    glDeleteShader(vertexShaderComp);
    glDeleteShader(fragmentShaderComp);

    // Return Shader Program
    return shaderProgram;
}

int main(void) {

    width = 640; height = 480;

    GLFWwindow* window;

    // Initialize the library
    if (!glfwInit())
        return -1;

    // Create a windowed mode window and its OpenGL context
    window = glfwCreateWindow(width, height, "Main Window", NULL, NULL);
    if (!window) {
        glfwTerminate();
        return -1;
    }

    // Make the window's context current
    glfwMakeContextCurrent(window);

    // Initialize GLEW
    if (glewInit() != GLEW_OK)
        cout << "Error!" << endl;

    GLfloat vertices[] = {

        // Triangle 1
        -0.5, -0.5, 0.0, // vert 0
        1.0, 0.0, 0.0,  // red

        -0.5, 0.5, 0.0, // vert 1
        0.0, 1.0, 0.0,  // green

        0.5, -0.5, 0.0,  // vert 2
        0.0, 0.0, 1.0,  // blue

        // Part of Triangle 2

        0.5, 0.5, 0.0,  // vert 3
        1.0, 0.0, 1.0  // purple

    };

    // Define element indices 
    GLubyte indices[] = {
        0,1,2,
        1,2,3
    };

    GLuint VBO, EBO, VAO;

    glGenBuffers(1, &VBO); // Create VBO and returns ID
    glGenBuffers(1, &EBO); // Create EBO

    glGenVertexArrays(1, &VAO); // Create VAO
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO); // Select VBO and activate buffer
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO); // Select EBO

    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW); // Load vertex attributes
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW); // Load indices attributes

    // Specify attributes location and layout to GPU
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)0);
    glEnableVertexAttribArray(0);

    // Color attribute location and layout
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)(3 * sizeof(GLfloat)));
    glEnableVertexAttribArray(1);

    glBindVertexArray(0);

    // Vertex shader source code
    string vertexShaderSource =
        "#version 330 core\n"
        "layout(location = 0) in vec4 vPosition;"
        "layout(location = 1) in vec4 aColor;"
        "out vec4 oColor;"
        "uniform mat4 model;"
        "void main()\n"
        "{\n"
        "gl_Position = model * vPosition;"
        "oColor = aColor;"
        "}\n";

    // Fragment shader source code
    string fragmentShaderSource =
        "#version 330 core\n"
        "in vec4 oColor;"
        "out vec4 fragColor;"
        "void main()\n"
        "{\n"
        "fragColor = oColor;"
        "}\n";

    // Creating Shader Program
    GLuint shaderProgram = CreateShaderProgram(vertexShaderSource, fragmentShaderSource);

    while (!glfwWindowShouldClose(window)) {

        // Resize window and graphics simultaneously
        glfwGetFramebufferSize(window, &width, &height);
        glViewport(0, 0, width, height);

        // Render here
        glClear(GL_COLOR_BUFFER_BIT);

        // Use Shader Program exe and select VAO before drawing
        glUseProgram(shaderProgram); // Call Shader per-frame when updating attributes

        // Declare identity matrix 
        glm::mat4 modelMatrix;

        // Initialize transforms
        modelMatrix = glm::translate(modelMatrix, glm::vec3(0.0f, 0.0f, 0.0f));
        modelMatrix = glm::rotate(modelMatrix, 45.0f, glm::vec3(0.0f, 0.0f, 1.0f));
        modelMatrix = glm::scale(modelMatrix, glm::vec3(0.5f, 0.5f, 0.5f));

        // Select transform and sent to shader
        GLuint modelLoc = glGetUniformLocation(shaderProgram, "model");

        glBindVertexArray(VAO); // User-defined VAO must be called before draw

        // Draw primitive(s)
        draw();

        // Unbind Shader exe and VOA after drawing per frame
        glBindVertexArray(0); // In case different VAO will be used after
        glUseProgram(0); // In case different shader will be used after

        // Pass transform to Shader
        glUniformMatrix4fv(modelLoc, 1, GL_FALSE, glm::value_ptr(modelMatrix));

        glBindVertexArray(VAO);

        // Swap front and back buffers
        glfwSwapBuffers(window);

        // Poll for and process events
        glfwPollEvents();
    }

    glfwTerminate();
    return 0;
}

I am trying to develop OpenGL code that uses shaders to render shapes. Within my Vertex Shader, whenever I add "uniform mat4 model;" and "gl_Position = model * vPosition;" my code stops rendering a shape. But when I remove "uniform mat4 model;" and take away "model from "gl_Position = model * vPosition;" it doesn't render anything. Why is this?