If my iMac isn't compatible with Catalina upgrade what do I do?

If you have a model that supports 10.13 High sierra or 10.14 Mojave you can update to that. You don;t need to install 10.15 Catalina if you don't want to.

See here: How to upgrade to macOS High Sierra - Apple Support

or here: How to upgrade to macOS Mojave - Apple Support

If you don't want Catalina there are still options for you.

Read on.

The model and year of your Mac will determine which macOS you can upgrade to,

Please use Safari to access the following links do not use Firefox

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

According to Microsoft's site, Sierra (what you have listed) is supported for Office 365 and 2019. So you don't need to do anything in regards to the OS.

Edit: That was somewhat older requirements I found. Microsoft's site now says this:

macOS: One of the three most recent versions of macOS. When a new major version of macOS is released, the macOS and the previous two versions.

Which puts Sierra out of the support period. If your requirements are to use the latest version of Office, then you need a new, or at least newer Mac.

Do you use a Virtual Machine?

If you end up using a Virtual Machine (Windows-in-a-window), you might want to consider creating a Virtual Machine to avoid this upgrade issue - Microsoft Office. Being affected by an upgrade of the MacOS.

Some people (such as myself) use a Virtual Machine(VM) for software and hardware that is Windows-Only compatible. You would open items in it, and, say, print right off of that for such a printer that does not operate with Mac.

As for Microsoft Office, you’d install the office version that pertains to that VM - thereby never having to worry about the MacOS upgrades getting in the way.

All would be accessed and would operate as desired on the VM. Virtual Machine applications include VMWare Fusion and Parallels. Learn more here: What Is a Virtual Machine? - VMWare Support.