Latest macOS upgrade available for my iMac 2015

yeah 2015 is too old for macOS Ventura is compatible with these computers - Apple Support

if it was me then I would try libreoffice Home | LibreOffice - Free and private office suite - Based on OpenOffice - Compatible with Microsoft

it's free and let you read and write the ms office formats

Alan Blair wrote:

Does Ventura come after Monterey?

Yes. With the release of Sequoia, Ventura is now the oldest of the "most recent three" that Microsoft supports.

FYI: mt MSOffice suite of products work just fine except that the Microsoft AutoUpdate app is where I got the message that I would need to upgrade to at least Mac OS 13 (whatever it is called).

macOS 13 is Ventura.

If you already are a Microsoft Office customer, and have it installed on Monterey, you can keep using It, but there won't be any new feature or security updates. If you aren't a Microsoft customer yet, want to be, but can't run at least Ventura, you are screwed. (Possibly you would also be screwed if you did a clean re-install of the operating system and then tried to do a clean re-install of Office or 365 on top of that. I don't know.)

Microsoft Support – Upgrade macOS to continue receiving Microsoft 365 and Office for Mac updates

Yes...