The CentOS-Base.repo is a standard repo, JetBackup plugin requires this repo to be enabled on your server for the plugin installation process to be successful.
Please refer to the link to manually install the repo: https://itectec.com/unixlinux/centos-how-to-install-the-stock-centos-repositories/
The CentOS-Base.repo is part of the centos-release.rpm which can be found here in this link: http://mirror.centos.org/centos-7/7/os/x86_64/Packages/centos-release-7-9.2009.0.el7.centos.x86_64.rpm
This website also gives information on installing the rpm: https://linuxhostsupport.com/blog/how-to-set-up-and-use-yum-repositories-on-centos-7/
Please check this with your qualified system administrator to enable/install the CentOS-Base.repo repo on your server.
