This Vagrant project uses Oscar to simplify the setup and config of the VM. So you'll need to install the oscar Vagrant plugin first:
$ vagrant plugin install oscar
The provisioning scripts for each VM install the latest Puppet 6 or 7 versions at the time you create the VM.
Puppet 7:
$ vagrant up server7
$ vagrant up agent7
Puppet 6:
$ vagrant up server6
$ vagrant up agent6