Android Registration form wont submit (Volley and SQL)

[nunu_psu]

Greetings everyone,

I'm still new to android programming and currently leanrning how to connect to DB.
I'm following a online tutorial but I'm still facing problems.
The tutorial is about creat a registration form and sending it to the database through volley.
I'm using .hostinger.ae as host and Android studio.
Everything seems fine until I click the register button it give me an error of (Couldn't register )
I couldn't locate the error so please I would appreciate any help

dbconnect.php

 <?php
define('HOST','mysql.hostinger.ae');
 define('USER','--');
 define('PASS','--');
 define('DB','u853541076_tuber');

 echo "access done";
 $con = mysqli_connect(HOST,USER,PASS,DB) or die('Unable to Connect');

and this is the volleyRegister

<?php

 if($_SERVER['REQUEST_METHOD']=='POST'){
 $username = $_POST['username'];
 $email = $_POST['email'];
 $password = $_POST['password'];

 require_once('dbConnect.php');

 $sql = "INSERT INTO volley (username,password,email) VALUES ('$username','$email','$password')";


 if(mysqli_query($con,$sql)){
 echo "Successfully Registered";
 }else{
 echo "Could not register";

 }
 }else{
echo 'error';
}

and this is my MainActivity.java

package myapp.com.tuberapp;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;

import org.json.JSONException;

import java.util.HashMap;
import java.util.Map;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

///The hosting link
    public static final String REGISTER_URL = "http://tuber.16mb.com/volleyRegister.php";

    public static final String KEY_USERNAME = "username";
    public static final String KEY_PASSWORD = "password";
    public static final String KEY_EMAIL = "email";

    private EditText editTextUsername;
    private EditText editTextEmail;
    private EditText editTextPassword;

    private Button buttonRegister;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        editTextUsername = (EditText) findViewById(R.id.editTextUsername);
        editTextPassword = (EditText) findViewById(R.id.editTextPassword);
        editTextEmail= (EditText) findViewById(R.id.editTextEmail);

        buttonRegister = (Button) findViewById(R.id.buttonRegister);

        buttonRegister.setOnClickListener(this);
    }

    private void registerUser(){
        final String username = editTextUsername.getText().toString().trim();
        final String password = editTextPassword.getText().toString().trim();
        final String email = editTextEmail.getText().toString().trim();

        StringRequest stringRequest = new StringRequest(Request.Method.POST, REGISTER_URL,
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {
                        Toast.makeText(MainActivity.this, response, Toast.LENGTH_LONG).show();
                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        Toast.makeText(MainActivity.this,error.toString(),Toast.LENGTH_LONG).show();
                    }
                }){
            @Override
            protected Map<String,String> getParams(){
                Map<String,String> params = new HashMap<String, String>();
                params.put(KEY_USERNAME,username);
                params.put(KEY_PASSWORD,password);
                params.put(KEY_EMAIL, email);
                return params;
            }

        };

        RequestQueue requestQueue = Volley.newRequestQueue(this);
        requestQueue.add(stringRequest);
    }

    @Override
    public void onClick(View v) {
        if(v == buttonRegister){
            registerUser();
        }
    }
}

What is it that I'm doing wrong?