1. You need to first make an image size (Name it Digital Download or something similar) in your price list with a unit size set to px.

2. Then you have to add that to your price list and check the box in your price list to make the image downloadable. You also need to check the box in your settings

3. Go under Settings>Gallery and check the box for Download Original Allow users to download original image if image size selected is not available.

Try those things and see if you get a download link.