报错： CentOS Linux 8 – AppStream Cannot prepare internal mirrorlist: No URLs in mirrorlist

解决：

1.将以下三个yum源文件中的 mirrorlist行注释掉，baseurl行打开，且将访问主机修改为腾讯云地址即可：

CentOS-Linux-AppStream.repo

CentOS-Linux-BaseOS.repo

CentOS-Linux-Extras.repo

具体如下：
/etc/yum.repo.d/CentOS-Linux-AppStream.repo

[appstream]
name=CentOS Linux $releasever - AppStream
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=AppStream&infra=$infra
#baseurl=http://mirror.centos.org/$contentdir/$releasever/AppStream/$basearch/os/
baseurl=http://mirrors.cloud.tencent.com/$contentdir/$releasever/AppStream/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

/etc/yum.repo.d/CentOS-Linux-BaseOS.repo

[baseos]
name=CentOS Linux $releasever - BaseOS
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=BaseOS&infra=$infra
#baseurl=http://mirror.centos.org/$contentdir/$releasever/BaseOS/$basearch/os/
baseurl=http://mirrors.cloud.tencent.com/$contentdir/$releasever/BaseOS/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

/etc/yum.repo.d/CentOS-Linux-Extras.repo

[extras]
name=CentOS Linux $releasever - Extras
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
#baseurl=http://mirror.centos.org/$contentdir/$releasever/extras/$basearch/os/
baseurl=http://mirrors.cloud.tencent.com/$contentdir/$releasever/extras/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

2.执行以下命令，清理和重新缓存源设置即可

yum clean all

yum makecache