Android Studio does not recognize R. Any Solution?

Question

package com.example.consult.myapplication;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

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

I wonder what error I have in this application that appears Cannot solves Symbol 'R' I just installed everything: Android Studio, Java and even Ubuntu. Note: JRE 1.8_121 | Ubuntu 16.04 LTS | Android Studio 2.3