iMac Need to upgrade to newer version

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

read Section 1 in each of the How To's.... below.

As has been established your mac can only upgrade as far as High Sierra,

if you are interested.

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

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 High Sierraapp.

Move it to the Trash, empty the Trash.

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

That machine can go as high as High Sierra. That's it. It can not run Catalina or Mojave.