[Solved] Could not estimate size of “Photos Library.photoslibrary”. iCloud Photos will not be enabled.

[Andy]

Suppose you already have iCloud enabled on another iPhone/iPad or macOS, and now you want to enable iCloud Photos on your Mac, and you get this error message:

Could not estimate size of “Photos Library.photoslibrary”. iCloud Photos will not be enabled.

Here's how to solve it:

Launch the Photos app, and then click on the menu bar: Photos -> Preferences... -> General.
Enable iCloud Photo Library in this panel, and make sure Use as System Photo Library is already enabled as well.

The option Use as System Photo Library should be grayed out.

Now we should be able to enable iCloud Photos (tick on the iCloud Photos), you can select Optimise Storage.

Now we have iCloud Photos enabled on your Mac.

Tested on: macOS Big Sur, version 11.5 (20G71)
If you have any questions or comments, you're more than welcome to post them down below