１．Error

# dnf update -y
CentOS Linux 8 - AppStream                      6.6  B/s |  38  B     00:05
Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist

２．mirrorlistのURLを書き換え

# sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-Linux-*
# sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-Linux-*

３．to upgrade to centos 8 stream:

# dnf install centos-release-stream -y
# dnf swap centos-{linux,stream}-repos -y
# dnf distro-sync -y