Upgrade Mac OS from catalina
System upgrade keeps spinning.
iMac 27″, macOS 10.12
Use About This Mac to determine what version of macOS you can upgrade to for your particular Mac.
It will give you a Model and Year that you can compare to the available versions, here:
How to download and install macOS - Apple Support
Under "Use the App Store," there are links to all of the available macOS versions.
I don't have links to the pages that show which Macs can run the individual macOS. If you post the model and year of your Mac, someone else may have the info on which OS you can use.