setSupportActionBar toolbar cannot be applied to (android.widget.Toolbar) error

In your Activity.java import android.support.v7.widget.Toolbar instead of android.widget.Toolbar:

import android.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.support.v7.widget.Toolbar;


public class rutaActivity extends AppCompactActivity {

private Toolbar toolbar;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_ruta);

    getSupportActionBar().hide();//Ocultar ActivityBar anterior

    toolbar = (Toolbar) findViewById(R.id.app_bar);
    setSupportActionBar(toolbar); //NO PROBLEM !!!!

Update:

If you are using androidx, replace

import android.support.v7.widget.Toolbar;
import android.support.v7.app.AppCompatActivity;

with newer imports

import androidx.appcompat.widget.Toolbar;
import androidx.appcompat.app.AppCompatActivity;

In using toolbar you should extends AppCompatActivity and

import android.support.v7.widget.Toolbar