Using centos 6 (with Plesk) after end of life (EOL)

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

Furthermore use need to update the source of your system packaged. This is a very good blob post about that: Thanks!

Just to be clear: Update as soon as possible to a modern system!