Need help!!! Update OS from 10.15.8 to 12 or higher!!!!

Sorry but Catalina is indeed the latest MacOS your iMac can run:

Ref:

Identify your iMac model - Apple Support

That model can't be upgraded past Catalina. Look for compatible versions of alternative web browsers.

(263893)

This is a link to a list of Macs compatible with macOS 11 Big Sur, the next version of the OS after Catalina.

macOS Big Sur is compatible with these computers - Apple Support

Every newer macOS since then will require a newer Mac.

If you are at all interested in buying a newer Mac, I highly recommend that you check the offerings at Refurbished Mac Deals - Apple. You'll get the same factory warranty and save a few dollars.

Since the web browser appears to be the primary problem here, the latest Firefox release is well worth testing, as Matti Haveri and Servant of Cats indicated.

https://www.firefox.com/en-US/firefox/151.0/system-requirements/

https://www.firefox.com/en-US/download/all/desktop-release/osx/

Some even older system versions can still use Firefox, but then an earlier Extended Support Release (ESR):

https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

Try if Firefox works with that website.

If the latest Firefox version doesn't work in Catalina, then at least Firefox 147.0.3 should work.

https://ftp.mozilla.org/pub/firefox/releases/

The current version of Firefox still supports Catalina. (Most other major third-party Web browsers now require Monterey or later.)