Catalina Update Issues.

My late 2015 iMac will not let me upgrade to Catalina. When I go into the app store and click on Catalina to "GET" it, I get the following message. "Update not found. The requested version of macOS is not available" . How do I get Catalina. I installed it on my MacBook.

Check System Preferences > Software Update or go to the App Store and search for Catalina.

I am trying to load Catalina on my Mac. It has frozen. I have rebooted the computer twice but it is still frozen. Any suggestions?

Catalina does not even show up as an option. It says my software is up to date on my late 2015 27" IMac