can I use an older version on Catalina?
@Tony Russell If you go to our Mac Compatibility article you will find a version of Kontakt 7 compatible with Catalina (7.3.2) and it's installer to download: Compatibility of Native Instruments Products on macOS
Usually download sales aren't refunded, but it's always worth putting in a support request. You can do that from here - Purchase & Account – Native Instruments (native-instruments.com)
can I download Kontakt 6 ? since I already paid for 7? I run OS Catalina
I don't think so as they're separate products. You need to contact support, or maybe @Jeremy_NI can help when he's online next.
Unfortunately - you are not entitled to access every prior version of a product simply because you bought the latest version.
And it goes without saying that you probably should have checked the system requirements before you purchased K7.
You could also consider the most logical thing and upgrade your OS first - problem solved.
Finally - K6 is now retired and is no longer available for purchase. And - many of the original libraries from NI that went with it - have now been updated to only work in K7.
K6 is a dead end regardless how you you want to frame it.
VP