Why is Catalina not available for my iMac 27" late 2013?

Any iMac from 2012 forwards can upgrade to Catalina provided it is currently running Mavericks (10.9) or later. You should see a red badge on System Preferences on your Mojave iMac, and in Software Update, the following should appear:

and here is the configuration for the Advanced… panel below the Upgrade Now button.

Unless you ran a Terminal command to gag Software Update from presenting any new operating system upgrades, then the preceding settings should produce the Catalina upgrade as in the first image above.

Have you applied all Mojave updates through macOS Mojave 10.14.6 Supplemental Update2 (18G1012)?

Run this command in the Terminal then reopen System Preferences > Software Update.

sudo softwareupdate --reset-ignored

What is the model ID of your Mac? Go to the  menu, hold down the Option key and select System Information. The ID no. will be under the Hardware/Hardware Overview section:

Mine is an iMac18,3.

But what is on that Advanced panel?

You're welcome.