How Do You Troubleshoot Vagrant when You Receive “The box you’re attempting to add doesn’t support the provider you requested”?

Problem scenario
You run "vagrant up", but you get one of the following error messages:

The box you're attempting to add doesn't support the provider
you requested. Please find an alternate box or use an alternate
provider. Double-check your requested provider to verify you didn't
simply misspell it.

If you're adding a box from HashiCorp's Vagrant Cloud, make sure the box is
released.

Name: foobar/Windows2016
Address: https://vagrantcloud.com/foobar/Windows2016
Requested provider: [:libvirt]
"

"The following settings shouldn't exist: provider"

What should you do?

Solution
Go to your Vagrantfile. Find the "provider" stanza. You need to have a stanza like this (but substitute "virtualbox" for "vmware_fusion" if necessary):

config.vm.provider "virtualbox"