I can't seem to download Catalina
I want to download Catalina but the app says I am not able to
iMac 27″, macOS 10.13
What iMac do you have only iMacs from 2012 and newer can upgrade to Catalina,
open Safari and click on this link, do not use Firefox.
How to upgrade to macOS Catalina – Apple Support
and look at Section 1.