I'd like to know : which Policy Server version supports installation and runninig of FSSUI on Apache 2.4 ?

Release:
Component: SMPLC

At first glance, only the Policy Server 12.6 offers that support. The
FSSUI gets deprecated in version more recent.

We support Apache 2.4 on RedHat 6 and 7 from Policy Server 12.6 only.

Legacy FSS Administrative User Interface

  | Web Server | Version | Windows Server | Red Hat Linux |
  |------------+---------+----------------+---------------|
  | ASF Apache | 2.4     |                | 7,6           |

  p.9

  Note that FSS does not have ability to manage features introduced
  since 12.0

https://support.ca.com/phpdocs/7/5262/5262_12_6_platform-support.pdf

You might also pay attention that FSSUI is now deprecated since 12.7 :

Deprecated FSS Administrative UI

  Starting release 12.7, the FSS Administrative UI is no longer shipped
  or supported.

  If you are performing an in-place upgrade, the
  NETE_PS_ROOT/siteminder/admin folder is deleted. If you have any files
  in that location, we recommend you back it up before upgrading.

  If you have configured the FSS UI to the following URL, it will no
  longer work as the FSS UI is now deprecated:

  http://<webserver>/siteminder/index.html

https://docops.ca.com/ca-single-sign-on/12-7/en/release-notes/changes-to-existing-features#ChangestoExistingFeatures-DeprecatedFSSAdministrativeUI

and some notes about the trouble shooting of FSSUI :

  FSSUI returns error 55
  https://comm.support.ca.com/kb/fssui-returns-error-55/kb000008406

  Access FSSUI : Applet not downloading.
  https://comm.support.ca.com/kb/access-fssui-applet-not-downloading/kb000007455

Please note that CA SSO 12.52 Policy Server, Admin UI and Access
Gateway will be end of service on February 28, 2019:

  https://support.ca.com/us/product-content/status/announcement-documents/2017/ca-single-sign-on-r12-52-end-of-service-announcement.html

We recommend upgrading to the latest version CA SSO 12.8, which offers
many new features, performance enhancements, and a new support
life-cycle until March 31, 2022 :

  https://support.ca.com/us/product-content/status/announcement-documents/2018/ca-single-sign-on-12-8-ga-announcement.html

A feature comparison datasheet is available for your review here :

  https://communities.ca.com/message/242107468-upgrade-from-ca-single-sign-on-v1252-to-v128