iOS enforces a strict permissions systems for access to the Photo Album; unless you explicitly grant access, PhotoRaw will not be able to import images. For historical reasons, iOS asks for permission to access "location services" when what it actually wants is access to the Photo Album. If you answer "No" to a request for this access, image imports will fail. You must answer "Yes" to enable PhotoRaw imports.
If you have already answered "No" to the Location Services prompt, see this page.