Hi,

You can't copy and paste photos from iCloud.com to Mac photo application , rather set up photo library and its preferences see the articles Manage your iCloud storage - Apple Support -- delete unwanted photos to reduce the iCloud storage space .

Set up and use iCloud Photos - Apple Support

Change preferences in Photos on Mac - Apple Support

System Photo Library overview for Photos on Mac - Apple Support

Is iCloud Drive and desktops box turned on see the articles Add your Desktop and Documents files to iCloud Drive - Apple Support

https://support.apple.com/en-in/guide/mac-help/mchle5a61431/10.14/mac/10.14

See this thread .

https://discussions.apple.com/thread/252150999

If folder is not synced with iCloud and located in some other location error will occur .

No, I meant I am copying from photos in my MacBook to a local folder. And sometimes it’s ok but usually it gives me unknown error 3.