I am trying to download 14.1 and it is telling me my system does not support it

Which exact model is your iMac? (It would have to be a Late 2012, or later). The (currently) latest version of macOS Mojave is 10.14.6. Is this what you are trying to update it to? If so, did you see any error messages other than the one you posted?

Again, you would need an iMac that is a Late 2012 model or later to upgrade past macOS High Sierra. Anything earlier would not be upgradable beyond that. Select the Apple logo in the left-hand side of the macOS menu bar, and then, select "About This Mac." That should provide you with the model.

Well there you go. That's the latest version of macOS that it can run.