Error while exporting the column from MySQL in Laravel through routing

I have Laravel project from Github, I am trying to export the selected column to a txt file. The table name is customers and the column name is customer_contact_numbers from the database khanoilsdb

Here is the error I am facing: I have added a new controller and the following code to it:

    <?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;

class CNController extends Controller
{
    $customers = Customer::all();
$phoneNumbers = "Phone numbers \n";
foreach ($customers as  $customer) {
  $content .= $customer->customer_contact_numbers;
  $content .= "\n";
}

// file name to download
$fileName = "contact_numbers.txt";

// make a response, with the content, a 200 response code and the headers
return Response::make($content, 200, [
  'Content-type' => 'text/plain', 
  'Content-Disposition' => sprintf('attachment; filename="%s"', $fileName),
  'Content-Length' => sizeof($content)
];);
}

Web.php:

Route::get('/home', 'CNController');

And the button:

      <div class="dropdown-menu dropdown-menu-right">
    <a class="dropdown-item" href="">Export</a>
  </div>

What I am trying to do is, after users click on it, it will call the controller method and start downloading a txt file.

Sorry if I'm not able to explain properly, I'm new to Laravel and PHP. Thanks :)

via Chebli Mohamed