OS Catalina

Yes, it is. See How to upgrade to macOS Catalina. You can download El Capitan here:

OS X El Capitan

Pick one or the other:

https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12

https://apps.apple.com/app/id1147835434?mt=12

Be sure to use Safari for any downloads.

You need to first download El Capitan. You also need to determine if your model can run Catalina. To find the complete model information select About This Mac from the Apple menu. You can create a snapshot of the dialog as follows:

How to take a screenshot on your Mac

Capture, Save or Record Screenshots in Mac OS X - Help Desk Geek

OS X Screen Capture Shortcuts

(⌘⬆︎3) Command-Shift-3 Capture the screen to a file

(⌘⬆︎⌃3) Command-Shift-Control-3 Capture the screen to the Clipboard

(⌘⬆︎4) Command-Shift-4 Capture a selection to a file

(⌘⬆︎⌃4) Command-Shift-Control-4 Capture a selection to the Clipboard

(⌘⬆︎5) Command-Shift-5 Invoke new screen grabber

You can then post the image in the forum's message editor.