Error: Failed to download metadata for repo 'AppStream'

The error is due to CentOS is already EOL and you need to update your repo so it points to the archived repo. You can do this by using the following sed command.

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

Once done, you should be able to do your yum update or dnf update.

CentOS - How to install kernel-debuginfo

First, check if you have the following this file (CentOS-Debuginfo.repo) in /etc/yum.repos.d

[root@abc yum.repos.d]# ls
CentOS-Base.repo  CentOS-Debuginfo.repo  CentOS-Media.repo

The content of CentOS-Debuginfo.repo is as shown below, from this file, we know the repo is debug (the name is enclosed with [ ]) and this repo is not enabled by default (enabled=0).

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#

# All debug packages from all the various CentOS-5 releases
# are merged into a single repo, split by BaseArch
#
# Note: packages in the debuginfo repo are currently not signed
#

[debug]
name=CentOS-6 - Debuginfo
baseurl=http://debuginfo.centos.org/6/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-Debug-6
enabled=0

To install kernel-debuginfo for your kernel, use yum as shown below

[root@abc yum.repos.d]# yum --enablerepo=debug install kernel-debuginfo-2.6.32-220.el6

If you omit kernel version, yum will pull the latest kernel-debuginfo file as shown below

[root@abc yum.repos.d]# yum --enablerepo=debug install kernel-debuginfo
Loaded plugins: fastestmirror, refresh-packagekit, security
Determining fastest mirrors
 * base: mirror.rndc.or.id
 * extras: kartolo.sby.datautama.net.id
 * updates: buaya.klas.or.id
base                                                     | 3.7 kB     00:00
debug                                                    | 1.9 kB     00:00
debug/primary_db                                         | 753 kB     00:19
extras                                                   | 3.5 kB     00:00
updates                                                  | 3.5 kB     00:00
updates/primary_db                                       | 1.5 MB     00:34
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package kernel-debuginfo.x86_64 0:2.6.32-358.2.1.el6.centos.plus will be installed
--> Processing Dependency: kernel-debuginfo-common-x86_64 = 2.6.32-358.2.1.el6.centos.plus for package: kernel-debuginfo-2.6.32-358.2.1.el6.centos.plus.x86_64
--> Running transaction check
---> Package kernel-debuginfo-common-x86_64.x86_64 0:2.6.32-358.2.1.el6.centos.plus will be installed
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
 Package                      Arch   Version                        Repository
                                                                           Size
================================================================================
Installing:
 kernel-debuginfo             x86_64 2.6.32-358.2.1.el6.centos.plus debug 249 M
Installing for dependencies:
 kernel-debuginfo-common-x86_64
                              x86_64 2.6.32-358.2.1.el6.centos.plus debug  38 M

Transaction Summary
================================================================================
Install       2 Package(s)

Total download size: 287 M
Installed size: 1.6 G
Is this ok [y/N]: N

Another option is to download both the rpm packages (kernel-debuginfo and kernel-debuginfo-common) from this link. (look for the correct architecture as well as kernel version)