Unable to install/update rpm with yum, once vm is created.

[llodiot]

I created vm, folowing "puppet for devops" class, using "vagrant up puppet --no-provision". Connnected on it as root, I'm not able to perform "yum -y update" with this kind of message : " puppet: CentOS Linux 8 - AppStream 26 B/s | 38 B 00:01
puppet: Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist"
Indeed editing yum repo files (in /etc/yum.repo.d) I saw that url includes variables which are not speciifed. As : "mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=AppStream&infra=$infra"
Note that creating vm with puppetserver installation (without option --no-provision) also failed for the same reason. Also note that I launch " vagrant plug in install vagrant-hostsupdater" before vm creation but it changes nothing.
Regards.