After Migration Assistant to New Computer, Can't Sync Photos Library to iCloud

I used Migration Assistant to move my files from an iMac to a MacBook Pro. I moved everything except the Pictures folder because my Photo Library was too large for this disk drive.

I have a new blank Photos Library, but I am not able to sync it with iCloud Photos. When I try to sync under System Settings, it asks for my computer password, and then says:

Open Photos to choose or create a library to use with iCloud Photos

I click the Open Photos ... button, and it takes me to Photos. But it never syncs, and the toggle is never set to On.

Any ideas on how to fix this? Thank you