Cannot find a valid baseurl for repo: centos-sclo-sclo/x86_64
出错的场景：在centos7机器上执行yum makecache或者使用yum命令安装软件时，出现下面的错误。

Loaded plugins: fastestmirror, product-id, search-disabled-repos, subscription-manager

This system is not registered with an entitlement server. You can use subscription-manager to register.

Cleaning repos: base centos-sclo-rh centos-sclo-sclo docker-ce-stable elrepo extras updates
Cleaning up list of fastest mirrors
Other repos take up 11 M of disk space (use --verbose for details)
Loaded plugins: fastestmirror, product-id, search-disabled-repos, subscription-manager

This system is not registered with an entitlement server. You can use subscription-manager to register.

Determining fastest mirrors
Could not retrieve mirrorlist http://mirrorlist.centos.org?arch=x86_64&release=7&repo=sclo-sclo error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
 One of the configured repositories failed (Unknown),
 and yum doesn't have enough cached data to continue. At this point the only
 safe thing yum can do is fail. There are a few ways to work "fix" this:

     1. Contact the upstream for the repository and get them to fix the problem.

     2. Reconfigure the baseurl/etc. for the repository, to point to a working
        upstream. This is most often useful if you are using a newer
        distribution release than is supported by the repository (and the
        packages for the previous distribution release still work).

     3. Run the command with the repository temporarily disabled
            yum --disablerepo=<repoid> ...

     4. Disable the repository permanently, so yum won't use it by default. Yum
        will then just ignore the repository until you permanently enable it
        again or use --enablerepo for temporary usage:

            yum-config-manager --disable <repoid>
        or
            subscription-manager repos --disable=<repoid>

     5. Configure the failing repository to be skipped, if it is unavailable.
        Note that yum will try to contact the repo. when it runs most commands,
        so will have to try and fail each time (and thus. yum will be be much
        slower). If it is a very temporary problem though, this is often a nice
        compromise:

            yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true

Cannot find a valid baseurl for repo: centos-sclo-sclo/x86_64

禁用无效的仓库。编辑CentOS-SCLo-scl.repo文件：

sudo vim /etc/yum.repos.d/CentOS-SCLo-scl.repo

将仓库禁用：

[centos-sclo-sclo]
name=CentOS-$releasever - SCLo sclo
baseurl=http://mirror.centos.org/centos/$releasever/sclo/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

保存文件。重新执行yum makecache。