Эта ошибка появляется в CentOS 8 из-за перехода репозитория на новый тип выпусков. Или что-то в этом роде. Что бы её побороть, надо переключиться на другой выпуск пакетов, а конкретно Stream, проявляется она вот так:

[root@hosting ~]# yum update
Failed to set locale, defaulting to C.UTF-8
Repository baseos is listed more than once in the configuration
CentOS Linux 8 - AppStream
Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in 
mirrorlist
[root@hosting ~]#

Для интереса можно зайти в каталог со списками репозиториев и посмотреть что там есть

[root@hosting ~]# cd /etc/yum.repos.d/
[root@hosting yum.repos.d]# ls
CentOS-Linux-AppStream.repo          CentOS-Linux-Debuginfo.repo  CentOS-Linux-FastTrack.repo         
CentOS-Linux-Plus.repo        CentOS-Stream-AppStream.repo  CentOS-Stream-Debuginfo.repo  CentOS-Stream- 
PowerTools.repo  CentOS-Stream-centosplus.repo  epel-testing-modular.repo  ispsystem-base.repo
CentOS-Linux-BaseOS.repo             CentOS-Linux-Devel.repo      CentOS-Linux-HighAvailability.repo  
CentOS-Linux-PowerTools.repo  CentOS-Stream-Base.repo       CentOS-Stream-Extras.repo     CentOS-Stream- 
Sources.repo     epel-modular.repo              epel-testing.repo          ispsystem.repo
CentOS-Linux-ContinuousRelease.repo  CentOS-Linux-Extras.repo     CentOS-Linux-Media.repo             
CentOS-Linux-Sources.repo     CentOS-Stream-BaseOS.repo     CentOS-Stream-Media.repo      CentOS-Stream- 
Vault.repo       epel-playground.repo           epel.repo

В моём случае там слишко много всего, видно что там есть подключенные репо и Linux и Stream. Попробуем установить пакет centos-release-stream-8.1, его можно скачать откуда угодно, но я использовал эту ссылку.

[root@hosting yum.repos.d]# rpm -i http://mirror.centos.org/centos/8/stream/extras/x86_64/os/Packages/centos-release-stream-8.1-1.1911.0.7.el8.x86_64.rpm
      package centos-release-stream-8.1-1.1911.0.7.el8.x86_64 is already installed
      

Пакет на самом деле стоит, но всё равно ничего не работает.

На всякий случай всё удаляем

[root@hosting yum.repos.d]# rpm -qa | grep centos-release-stream
[root@hosting yum.repos.d]# rpm -e centos-release-stream-8.1-1.1911.0.7.el8.x86_64

И устанавливаем заново

[root@hosting yum.repos.d]# rpm -ivh http://mirror.centos.org/centos/8-stream/extras/x86_64/os/Packages/centos-release-stream-8.1-1.1911.0.7.el8.x86_64.rpm
Retrieving http://mirror.centos.org/centos/8-stream/extras/x86_64/os/Packages/centos-release-stream-8.1-1.1911.0.7.el8.x86_64.rpm
Verifying...                          ################################# [100%]
Preparing...                          ################################# [100%]
Updating / installing...
   1:centos-release-stream-8.1-1.1911.################################# [100%]
 

Пакет поставлен, теперь переключаемся между релизами.

[root@hosting yum.repos.d]# dnf swap centos-{linux,stream}-repos -y
Failed to set locale, defaulting to C.UTF-8
Repository baseos is listed more than once in the configuration
CentOS Linux 8 - AppStream
Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist

Получаем ошибку. Что бы её полечить надо отключить предыдущий репозиторий AppStream

[root@hosting yum.repos.d]# vi CentOS-Linux-AppStream.repo

Ищем там

enabled=1

И меняем на

enabled=0

Сохраняем, выходим, запускаем ещё раз

[root@hosting yum.repos.d]# dnf swap centos-{linux,stream}-repos -y
Failed to set locale, defaulting to C.UTF-8
Repository baseos is listed more than once in the configuration
CentOS Linux 8 - BaseOS
Error: Failed to download metadata for repo 'baseos': Cannot prepare internal mirrorlist: No URLs in mirrorlist

Теперь отключаем так же другой репозиторий

[root@hosting yum.repos.d]# vi CentOS-Linux-BaseOS.repo

Запускаем ещё раз

[root@hosting yum.repos.d]# dnf swap centos-{linux,stream}-repos -y
Failed to set locale, defaulting to C.UTF-8
Repository baseos is listed more than once in the configuration
CentOS Linux 8 - Extras
CentOS Linux 8 - PowerTools
Error: Failed to download metadata for repo 'powertools': Cannot prepare internal mirrorlist: No URLs in mirrorlist

И ещё раз выключаем третий репо

[root@hosting yum.repos.d]# vi CentOS-Linux-PowerTools.repo

Обычно этого достаточно, дальше запускаем и оно всё отрабатывает

[root@hosting yum.repos.d]# dnf swap centos-{linux,stream}-repos -y
Failed to set locale, defaulting to C.UTF-8
Repository baseos is listed more than once in the configuration
CentOS-Stream - AppStream
CentOS-Stream - Base
CentOS-Stream - Extras
Extra Packages for Enterprise Linux 8 - x86_64
Extra Packages for Enterprise Linux Modular 8 - x86_64
ispsystem-5.279
ispsystem-base
Dependencies resolved.

===========================================================================================================
 Package Architecture Version Repository Size

 ===========================================================================================================
Installing:
 centos-stream-release noarch 8.6-1.el8 Stream-BaseOS 22 k
     replacing  centos-linux-release.noarch 8.5-1.2111.el8
     replacing  centos-release-stream.x86_64 8.1-1.1911.0.7.el8
 centos-stream-repos noarch 8-4.el8 Stream-BaseOS 20 k
Upgrading:
 centos-gpg-keys noarch 1:8-4.el8 Stream-BaseOS 12 k
Removing:
 centos-linux-repos noarch 8-3.el8 @baseos 26 k

Transaction Summary

===========================================================================================================
Install  2 Packages
Upgrade  1 Package
Remove   1 Package

Total download size: 54 k
Downloading Packages:
(1/3): centos-gpg-keys-8-4.el8.noarch.rpm
(2/3): centos-stream-release-8.6-1.el8.noarch.rpm
(3/3): centos-stream-repos-8-4.el8.noarch.rpm
-----------------------------------------------------------------------------------------------------------
Total                                                                                                                                                                                                                                                                                                           
39 kB/s |  54 kB     00:01
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Preparing        : 1/1
  Running scriptlet: centos-gpg-keys-1:8-4.el8.noarch 1/1
  Upgrading        : centos-gpg-keys-1:8-4.el8.noarch 1/7
  Installing       : centos-stream-repos-8-4.el8.noarch 2/7
  Installing       : centos-stream-release-8.6-1.el8.noarch 3/7
  Obsoleting       : centos-release-stream-8.1-1.1911.0.7.el8.x86_64 4/7
  Obsoleting       : centos-linux-release-8.5-1.2111.el8.noarch 5/7
  Erasing          : centos-linux-repos-8-3.el8.noarch 6/7
warning: /etc/yum.repos.d/CentOS-Linux-BaseOS.repo saved as /etc/yum.repos.d/CentOS-Linux-BaseOS.repo.rpmsave
warning: /etc/yum.repos.d/CentOS-Linux-AppStream.repo saved as /etc/yum.repos.d/CentOS-Linux-AppStream.repo.rpmsave

  Cleanup          : centos-gpg-keys-1:8-3.el8.noarch 7/7
  Running scriptlet: centos-gpg-keys-1:8-3.el8.noarch 7/7
  Verifying        : centos-stream-release-8.6-1.el8.noarch 1/7
  Verifying        : centos-linux-release-8.5-1.2111.el8.noarch 2/7
  Verifying        : centos-release-stream-8.1-1.1911.0.7.el8.x86_64 3/7
  Verifying        : centos-stream-repos-8-4.el8.noarch 4/7
  Verifying        : centos-gpg-keys-1:8-4.el8.noarch 5/7
  Verifying        : centos-gpg-keys-1:8-3.el8.noarch 6/7
  Verifying        : centos-linux-repos-8-3.el8.noarch 7/7

Upgraded:
  centos-gpg-keys-1:8-4.el8.noarch
Installed:
  centos-stream-release-8.6-1.el8.noarch                                                                                                                                   
  centos-stream-repos-8-4.el8.noarch
Removed:
  centos-linux-repos-8-3.el8.noarch

Complete!

попутно исправим дурацкую ошибку с локалью

[root@hosting /]# dnf install langpacks-en glibc-all-langpacks -y