How to fix common CentOS error codes?

There are a few common error codes that can be seen when using Centos, these are:

1. yum update errors
2. dnf update errors
3. rpm new key errors
4. "cannot find a valid baseurl for repo" errors
5. "Error: Cannot retrieve repository metadata (repomd.xml) for repository: epel. Please verify its path and try again" These error codes can generally be caused by a few things, the most common being incorrect repository configuration, incorrect DNS settings, or a proxy/firewall blocking access to the repository.