I am using the free version on Windows 11. Any idea why Duplicate Finder would not delete the files selected for deletion ? The duplicates are in the same folder as the originals. After clicking on "Delete Selected" it looks like it's going to delete them, but in the end a message will appear saying "The operation has been completed. Some of the selected files were not deleted".
I'd suspect the usual Windows reasons for not deleting them, eg. File open in something, File needs Admin permission to delete.
Make a note of one of the ones it wouldn't delete (after deleting run the same search again to see which those were) and try deleting it in File Explorer, that should give you a clue what the problem is.