Hi Eric,
thanks for your fast recommendations.
I checked the application compatibility list: OK
I also checked El Capitan compatibility information: OK
I have checked Sierra 10.12. compatibility apps: OK
I have installed printer driver for MacOS 10.11: OK
But the download of El capitan failed with the same message as i wrote Niels before:
(which means that this version of OS X 10.11 can not be installed on this computer, no details if you click on "more information")

So far I´m frustrated because i bought Apple products with "easy to use" label on each Apple product.
That´s what everybody thinks about Apple products.
But now i spend almost Friday, Saturday and Sunday (with low technical background) to get this update done and it does not work.
What do you think of my new idea to set up a windows PC with latest iTunes software beside my MacBook to run the iPhone 6 and iPad backups on the windows PC?
Best regards from Germany, Manfred