Problem inserting setTitle ("title");

I can not believe I can not put a simple Toolbar, my code is as follows:

MainActivity:

package com.example.a60393.calculadorakuspit;

    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.support.v7.widget.Toolbar;
   public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

  Toolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    toolbar.setTitle("Calcula el Total");  }}

toolbar

    <?xml version="1.0" encoding="utf-8"?><android.support.v7.widget.Toolbar
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:elevation="4dp"
android:background="@color/colorPrimary"
android:popupTheme="@style/ThemeOverlay.AppCompat.Light"
android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">
</android.support.v7.widget.Toolbar>

activity_main

<?xml version="1.0" encoding="utf-8"?>

<include
    android:id="@+id/toolBar"
    layout="@layout/toolbar"/>

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerInParent="true"
    android:text="Hello World!"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

  </RelativeLayout>