It's likely your permissions (http://laravel.com/docs/5.1#basic-configuration) If you are using Mac you should be able to go to the folder in Terminal. In Terminal:
chmod 755 -R /full/path/to/your/laravel-app/storage
I'm not sure if you'd have the same issue on Windows. If you have MAMP set up with FTP just FTP in with something like FileZilla and right click the storage folder -> Change permissions to 755.