I can't find software updates in system preferences

There are no more Sierra Updates as Apple stopped Security Updates to Sierra several year ago. If what you really meant was upGrade - that involves moving from Sierra to a newer version of macOS. Understand, the Age of this computer will Govern what newer version it may Qualify to install. About this Mac will provide the needed information. Follow this link How to get old versions of macOS and verify what computers qualify for what macOS and compare that against your computer age.

Once you have done that, you can commence the download using ONLY SAFARI as Others may not work. Safari will open a redirect and open the Apple Store where you can download the Qualifying version of macOS.

Notations - Make a Time Machine Backup Before attempting the upgrade. Also, be aware- starting in Catalina and Big Sur, there is No Support for Anything 32 bit and there is not workaround.

It's under the Apple menu, not in System Preferences.

I caught on all that too but didn't bother posting about it because although the OP posted this in the High Sierra forum, the hardware profile selected suggests Sierra as the OS being used. Since the hardware profiles do not permit a particular version of an OS to be selected (e.g. 10.13.6) it is possible a machine really could be updated and not just upgraded. Anyway, the real issue seemed to be not seeing Software Update, regardless of the OS in question. :-)

Valid points. Though, one would/ could believe, by now, the High Sierra Security update November 2020 would have drop onto the machine by now ?

Yes, but some people (like me) like to know what and when things are done to their hardware and have disabled automatic updates.

Again Valid point- Actually thought of that after posting but alas - too late to amend. Thanks