App.txt messages do not appear

I cannot figure out how to get his moved to the section Building Websites with PHP-->Namespacing

I followed some advice as I had an earlier issue with the logger and in doing so deleted my app.txt and then created a new app.txt. FWIW: The app.txt had messages in it but since I deleted it, it no longer does.

Now this is the error I am getting

Fatal error: Uncaught exception 'UnexpectedValueException' with message 'The stream or file "app.txt" could not be opened: failed to open stream: Permission denied' in /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php:87 Stack trace: #0 /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Handler/AbstractProcessingHandler.php(37): Monolog\Handler\StreamHandler->write(Array) #1 /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Logger.php(289): Monolog\Handler\AbstractProcessingHandler->handle(Array) #2 /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Logger.php(341): Monolog\Logger->addRecord(300, 'Foo', Array) #3 /home/treehouse/workspace/index.php(13): Monolog\Logger->addWarning('Foo') #4 {main} thrown in /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php on line 87

This is the code I have

<?php

require __DIR__ . '/vendor/autoload.php';

date_default_timezone_set ('America/New_York');

use Monolog\Logger;
use Monolog\Handler\StreamHandler;

$log = new Logger('name');
$log->pushHandler(new StreamHandler('app.txt', Logger::WARNING));

$log->addWarning('Foo');

echo "hello, world";


?>

It is not a browser error as I have cleared the cache, this is what my file tree looks like http://pasteboard.co/2yN9nF7C.png, and I have refreshed my file tree.