UdacityのLinux Command Line Basics
のLesson 1のセットアップをしていたら、以下のエラーが出ました。

VirtualBoxを最新6.0.0にしたら解決しました。

$ vagrant up

No usable default provider could be found for your system.

Vagrant relies on interactions with 3rd party systems, known as
"providers", to provide Vagrant with resources to run development
environments. Examples are VirtualBox, VMware, Hyper-V.

The easiest solution to this message is to install VirtualBox, which
is available for free on all major platforms.

If you believe you already have a provider available, make sure it
is properly installed and configured. You can see more details about
why a particular provider isn't working by forcing usage with
vagrant up --provider=PROVIDER, which should give you a more specific
error message for that particular provider.