Can you upgrade to your Mac to High Sierra or is your current El Capitan the last version your Mac can run?
High Sierra supports more cameras: Digital camera RAW formats supported by iOS 11 and macOS High Sierra - Apple Support. If your Mac can be upgraded, and High Sierra supports your camera, consider to upgrade, but check, if the new system version supports all expensive software and device drivers you need, before you take the plunge.
Nikon offers software for the RAW developement. You will usually get the best possible raw development if you use the RAW converter that came with your Camera.
The Adobe RAW converter is free: https://supportdownloads.adobe.com/product.jsp?product=106&platform=Macintosh
Is your camera on the list of supported cameras? If yes, use it to convert the RAW to DNG files.