I Can't Manually Add A Face

How are you trying to add a face? And what does not work?

To add a face,

• you double-click the thumbnail of the photo to view it enlarged.
• If it is not enabled, enable "View > Show Face names"
• Then enter ⌘I to open the Info.
• In the Info click the button (+) Add Faces.
• Now move the new faces circle to to position where you want it and drag the resize hand to adjust the size.
• Type a name into the name field below the circle.

If this does not work, because the circle cannot be moved or the size is wrong, quit Photos and restart it. occasionally I had even to restart the Mac to be able to add a new face.