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That will depend on whether your iMac is a late-2012 or newer model, which you can determine from the menu > About This Mac panel. Look at what is inside the parenthesis on your product description line from that panel, and use that information to determine your hardware compatibility in How to get old versions of macOS, which you would exclusively use the Safari browser in that link.

Understand the Catalina only supports 64-bit applications, printer, and scanner drivers that are stated by their vendors to be compatible with it. You may want to run the free Go64 application to obtain a list of your existing 32-bit applications, and also check your printer/scanner vendor websites for drivers more current than Oct 2019. Never perform an operating system upgrade without a Time Machine backup of the old system.

If you have a 2012 or later Mac…

Open in Safari…

https://itunes.apple.com/us/app/macos-catalina/id1466841314?ls=1&mt=12

Otherwise, no.