vagrant upを実行すると以下のメッセージが表示されます。
OS:Windows8です。

PS C:\vboxvms\mycentos> vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'beto/centos-6.8' could not be found. Attempting to find and install...
default: Box Provider: virtualbox
default: Box Version: >= 0
The box 'beto/centos-6.8' could not be found or
could not be accessed in the remote catalog. If this is a private
box on HashiCorp's Vagrant Cloud, please verify you're logged in via
`vagrant login`. Also, please double-check the name. The expanded
URL and error message are shown below:

URL: ["https://vagrantcloud.com/beto/centos-6.8"]
Error: The requested URL returned error: 404 Not Found