If you get the above error in “Photos Duplicate Cleaner” for mac, chances are the folder you selected in in the ignore list. Probably you selected one of the folder on Volume and volume (external drive) is automatically included in ignore list. The easiest fix remove all locations in ignore directories by going to preferences and then select – button repeatedly. Select your folder, this time you should get no error. At this point you might find why you were getting this error. Once done simply click restore default.
The ignore list is actually quite useful in itself. It will not look into ignore directories. Good to add some safe locations in ignore list such as Application folder.