iCloud NOT Working on OS X Sierra

First, remove any VPN add-on apps and any add-on security apps and any add-on cleaner apps, and reboot into Safe Mode, and try again. This removes some potentially-problematic apps, and also rebuilds caches and settings.

How to use safe mode on your Mac - Apple Support

Which iMac 27” model year? Given you’re on 10.12, it’ll upgrade to at least 10.13, and quite possibly newer.

How to get old versions of macOS - Apple Support

Get to at least Mojave if you can, and preferably to more current, if you have 64-bit apps and a less-old model year of iMac.

I’ve also seen reports of either an expired certificate or a problem with certificate pinning possibly involved in an iCloud outage from High Sierra and earlier. That outage can potentially be remedied.

If so, here’s a potential work-around:

Unable to establish secure connection to … - Apple Community

Also see: How I solved the Safari 13 and High Sierr… - Apple Community

Check this here: https://discussions.apple.com/thread/251211674

Updating the immediate cert fixed this for me.

Thank you! I solved the issue too. I noticed however that the certificate is going to expire on May, 20th 2022.

Am I going to struggle with this issue again when the certificate expires?