If you are in need to install some software on a (very) system, these may helps:
First of all use an archive version of plesk packaged:
plesk installer --source autoinstall-archives.plesk.com
Furthermore use need to update the source of your system packaged. This is a very good blob post about that: https://gcore.de/en/help/linux/centos6-new-repo-url-after-eol.php Thanks!
Just to be clear: Update as soon as possible to a modern system!