I am unable to install macOS Monterey on to my iMac

Select the Apple () menu : About This Mac panel. The product description line will have your Mac model and year in parenthesis. Compare that to the hardware supported by macOS Monterey or Ventura. Use the following link with Safari to get the latest of the two that your hardware supports:

How to download and install macOS - Apple Support

The macOS installer will be downloaded from the Mac App Store to your /Applications folder, and may or may not initially launch. I would quit that if it launches, and make certain you have a last Time Machine backup of Catalina before performing the upgrade.

If your iMac is not supported by either, then you will be looking for a MS Office clone instead of MS Office. That might be the free LibreOffice Suite, or the paid (free trial) Softmaker's Office 2024 for Mac.

Yes, I think you got it...

Monterey…

macOS Monterey is compatible with these computers - Apple Support

• iMac – all models from late 2015, iMac 16,1 – 20,2
• iMac Pro – all models, iMac Pro 1,1
• MacBook & MacBook Retina – all models from early 2016, MacBook 9,1 – 10,1
• MacBook Pro – all models from early 2015, MacBook Pro 12,1 – 17,1
• MacBookAir – all models from early 2015, MacBook Air 7,1 – 10,1
• Mac Mini – all models from 2014, Mac Mini 7,1 – 9,1
• Mac Pro – all models from 2013, Mac Pro 6,1 – 7,1

https://wpbeaches.com/apple-minimum-system-requirements-for-macos-monterey-12/

https://apps.apple.com/us/app/macos-monterey/id1576738294

macOS Monterey on the App Store (apple.com)

https://apps.apple.com/us/app/macos-monterey/id1576738294?mt=12

Monterey full installer.app

Why does OSX Monterey 12gb download turn … - Apple Community