dnf and yum: can not find package

yum repolist will display the active repo list, I suspect the packages your looking for are not in the base/update/extras repositories and you may need to add additional repositories.

A good way to find out is to google search the package your looking for to get an idea of repository you need to have setup or install.

A lot of repositories do have a RPM file that will install the repository for your or a "how to" for adding the repository.

Examples below

Red Hat has made the documentation free to read
9.5.2. Setting [repository] Options

IUS repo setup
IUS Getting Started

yum repolist example:

Loaded plugins: fastestmirror, ovl
Determining fastest mirrors
 * base: mirror.its.sfu.ca
 * extras: mirror.it.ubc.ca
 * updates: centos.mirror.rafal.ca
base                                                                                                                                                                                                                  | 3.6 kB  00:00:00     
extras                                                                                                                                                                                                                | 3.4 kB  00:00:00     
updates                                                                                                                                                                                                               | 3.4 kB  00:00:00     
(1/4): base/7/x86_64/group_gz                                                                                                                                                                                         | 166 kB  00:00:00     
(2/4): extras/7/x86_64/primary_db                                                                                                                                                                                     | 150 kB  00:00:00     
(3/4): updates/7/x86_64/primary_db                                                                                                                                                                                    | 3.6 MB  00:00:00     
(4/4): base/7/x86_64/primary_db                                                                                                                                                                                       | 5.9 MB  00:00:01     
repo id                                                                                                           repo name                                                                                                            status
base/7/x86_64                                                                                                     CentOS-7 - Base                                                                                                      9911
extras/7/x86_64                                                                                                   CentOS-7 - Extras                                                                                                     314
updates/7/x86_64                                                                                                  CentOS-7 - Updates                                                                                                    946
repolist: 11171

Have you tried yum search packageName ?

I have seen similar behavior from two other causes, DNS is not working or the cache is corrupted.

On that machine can you ping a host by name? i.e. ping google.com ?

If that works, try a yum clean all and then try to install your package when it is done.