Is your computer compatible with Sierra or High Sierra? That is what usually causes that message.

How to get macOS High Sierra

Sierra download

Check to make sure your applications are compatible.     

Application Compatibility

El Capitan 10.11 Compatibility information

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

Security Update 2018-001 El Capitan

Kernel

Available for: macOS Sierra 10.12.6, OS X El Capitan 10.11.6

Impact: An application may be able to read kernel memory (Meltdown)

Description: Systems with microprocessors utilizing speculative execution and indirect branch prediction may allow unauthorized disclosure of information to an attacker with local user access via a side-channel analysis of the data cache.

CVE-2017-5754: Jann Horn of Google Project Zero; Moritz Lipp of Graz University of Technology; Michael Schwarz of Graz University of Technology; Daniel Gruss of Graz University of Technology; Thomas Prescher of Cyberus Technology GmbH; Werner Haas of Cyberus Technology GmbH; Stefan Mangard of Graz University of Technology; Paul Kocher; Daniel Genkin of University of Pennsylvania and University of Maryland; Yuval Yarom of University of Adelaide and Data61; and Mike Hamburg of Rambus (Cryptography Research Division)

How to Downgrade macOS High Sierra

macOS Reversion- How to Downgrade from High Sierra

If you downloaded El Capitan in the past then you can re-download it from the Purchases page in App Store. Otherwise, App Store won't permit a fresh download on a machine running a later version of macOS.

Get El Capitan from the App Store not through software update How to download OS X El Capitan - Apple Support

I would uninstall Intego. It tends to interfere with the computer's operation while providing minimal to no benefit unless you work with Windows files.

Intego Uninstall

Here are the system requirements for High Sierra. If your computer does not meet these requirements you can not install High Sierra.

macOS High Sierra - Technical Specifications

Thanks for all answers.

I managed it this way: Restored the system again, installed Java 8 supplied by Oracle, placed fake Java 6 docs at the place CS6 expects them and Finder, Illustrator, Photoshop runs again. I am happy!

Only if it meets the system requirements for High Sierra.

(157557)

OK, me again!

It looks like App Store "remembered" my original El Capitan download for another machine, eons ago. I should have used the "Purchased" button in App Store. Looks like problem has been solved. Anyway, thanks to those you took the time to read my original post.

Paul

We were all correct, you, ckuan, and me. I back pedalled by re-installing my previous version El Capitan V10.11.5, and my Safari reversed to V9.1.1. That indicated this Safari version to be part of the OS installation.

I then upgraded again to OS X V10.11.6, which also included Safari V9.1.2, part of OS installation.

At this stage I had never seen a separate offer on the App Store for a Safari update, and a search did not come up with any result either.

I then installed the security upgrade 2018-001 for El Capitan (there are several security upgrades by that number for other systems). After that installation, the separate Safari upgrade for V11.0.3 showed up in the App Store. I installed it to bring my system up todate.

Thanks for all your input.