@Ddanielson @Denny Okay, according to Apple's support document, iCloud+ requires Monterey, so I guess that makes sense. But maybe, as @maique mentions, the extra storage part of iCloud+ works anyway, and it's the other services, like HomeKit Secure Video, that won't work?
If you're up for an adventure, you could probably upgrade to Monterey via the OpenCore Legacy Patcher project.