Problem running gulp

My Gulp worked normally, from one hour to another started to present this error:

The file /var/www/html/dna/public/css/app.css is allowed 777.

Contents of the gulpfile.js file

var elixir = require('laravel-elixir');

elixir(function(mix) {

    mix.sass('app.scss');
    mix.styles(['normalize.css', 'flatcolors.css'], 'public/css/padrao.css')
        .styles(['bootstrap.min.css'], 'public/css/bootstrap.css');
    mix.scripts([   'jquery-2.1.4.js', 
            'bootstrap.min.js', 
            'functions.js', 
            'ajax.js', 
            'jquery.blockUI.js'
    ]);

});