I checked some more, turns out it's shown for (a) few photos, mostly where half or more of the photo is the sky, though I wasn't able to get the button to show on a photo cropped to be mostly sky. So if you want it for any specific photo, your best b...

I know it's an old question but if you haven't still found it (or for anyoje searching), it is in the new Galaxy Enhance-X app, I don't see it for any of my photos in the gallery app.