Photos won't upload to iCloud

In photos on your Mac, go to the Library view and scroll down to the bottom. You'll find you current album content quantity. Beneath that is the iCloud status.

You may find that it takes some time to upload the initial iCloud album depending on the size of the album. I recently helped a family member with an album or more than 30,000 images and the initial upload took a few days (of course, they were using the computer, taking it out of the house, etc.)

This document serves as a general guide: https://support.apple.com/guide/photos/from-icloud-photos-phtf5e48489c/mac

I suspect that in order to sync there has to be a minimum amount of free space on your device. I found an old thread: https://discussions.apple.com/thread/6983519

I'd check how much space is available on your Mac. And, remember that iCloud is a conduit, not an offline storage mechanism. So your device remains in sync with iCloud if pictures/files/etc. are deleted across devices.

Did a little more digging too. It seems some folks have been able to toggle iCloud photos on and off and it will work. A few other posters simply left their device plugged in overnight and the message went away and the sync started.