There are two parts to answering this question:
You need to extract metadata from your images to use this feature. Select an option from “Extract metadata from where?” and fill in the requisite details.
After you’ve done the above, the duplicate/missing files setting is actually hidden away under the “Group images by metadata” setting that should now have appeared. Check that box and the duplicate/missing files setting will also appear.
The duplicate/missing images setting needs a way to find metadata, which we define as corresponding matching text in your image/channel filenames. If you don’t have metadata in your images, then unfortunately this feature won’t work for you.