iMac is saying there is no software update for MacOS

Hi, my 21.5 inch iMac 2017 isn't letting me update OS.

I am running on High Sierra and want to update to the latest update however when I click the  and then go to about this Mac, then software update, there is no MacOS software update showing, it is just showing my app updates, which I'm unable to do as my software is incompatible for them. I'm not sure as I've ever really done many updates for my Mac since I've had it, but I managed to update it to High Sierra a couple of days ago, however it is not showing any other updates.

Can anyone help me update to the latest version of OS?