You can invert the qualifier result with the button on the right of the Selection Range tools:

- invert qualifier.jpg (144.47 KiB) Viewed 819 times
You may also need a garbage matte to target the areas not covered by the green screen.
Also, before Jim gets here

, many people seem to miss that Blackmagic offers a good range of tutorials and pdf-books on Resolve for free:
https://www.blackmagicdesign.com/products/davinciresolve/trainingAnd for my favorite tutorials, search for "Goats eye view" on Youtube. She covers this type of situation too, iirc.