sudo yum install dnf-plugins-core did the trick for me.

UPDATE: on el7, el8, el9 there is a better way

• when you try to execute dnf config-manager -h it will tell you to try dnf install 'dnf-command(config-manager)'

# centos7
# rocky8
# rocky9
sudo dnf install 'dnf-command(config-manager)'

p.s. the high level solution is actually "try to execute the dnf config-manager -h and if the command fails then read the error message and try installing what the error message recommends".

I hesitate to add this as an answer because I'm not sure it's a good way to fix it.. But anyway I had the same issue and 'fixed' it by adding module_hotfixes=true in /etc/dnf/dnf.conf.

https://dnf.readthedocs.io/en/latest/conf_ref.html tells us:

module_hotfixes

boolean

Set this to True to disable module RPM filtering and make all RPMs from the repository available. The default is False. This allows user to create a repository with cherry-picked hotfixes that are included in a package set on a modular system.

I'm not sure why this works in this case, I only have dnf on some sandbox servers and never got to the bottom of it.

There are multiple arguments for the thesis that it is not supported:

Here's what I found:

• The dnf GitHub project's README provides info how to install it only for Fedora and states "In other RPM-based distributions you need to build all the components from their sources.".

  • ...but it in fact the dnf RPM package for Centos 7 is available in the Centos extras repo, but it is v. 4.0.9, from Nov 2018 (appropriate dnf changelog entry), while the current latest version is 4.5.2 (from Dec 2020).

• This dnf FAQ entry says that it is safe to install dnf next to yum and provides a tool for migrating the history from yum to dnf ...but the command provided did not work for me as there is no such package as python-dnf-plugins-extras-migrate - this plugin is in the dnf-plugins-core package instead. But after running it with dnf migrate it errored with RuntimeError: Exec failed: no such table: main.trans_cmdline...

• The Centos wiki page about using dnf on Centos 7 as yum4 - https://wiki.centos.org/SpecialInterestGroup/ConfigManagementSIG/YUM4 - is not up to date as it is referring to Centos 7.5, released in 2018.

• On the Centos blog the latest info about this is an article from 2018 - https://blog.centos.org/2018/04/yum4-dnf-for-centos-7-updates/ and the last question there - "Is it safe to use yum4/dnf on CentOS 7?" - has been unanswered for more than 1 year (since Dec 2019)...

...on the other hand, there are also arguments for the thesis that it is supported and maybe that support is just well not documented and updated (in a typical open source project way 🙄)?

Arguments for:

• RedHat Bugzilla does contain bug reports from users of dnf on Centos 7 and RedHat employees did work on them.