Uncomment baseurl in this file: /etc/yum.repos.d/CentOS-Base.repo
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
In this block:
name=CentOS-$releasever - Base mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra #baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/ gpgcheck=1 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
By uncomment, I mean change this:
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/, to this: baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/ (remove the #).