vagrant up has been finished successfully, but after that all subsequent commands like
vagrant halt are failed with an error:
The provider 'Parallels' could not be found, but was requested to back the machine 'default'. Please use a provider that exists.
It’s a known issue of Vagrant on OS X.
It might happens you have typed the provider name incorrectly, for example, like that:
vagrant up --provider=Parallels
In this case Vagrant creates a data directory
.vagrant/machines/default/Parallels which will not be removed completely even after Vagrant failure or virtual machine destroy.
Rename a Vagrant machine’s data directory to the lower case: from “Parallels” to “parallels”, for example:
mv .vagrant/machines/default/Parallels .vagrant/machines/default/parallels
For more information please follow to this tread on Parallels Forum