RAW editing option not available

Do you mean the command "Image > Use RAW as Original"? This command is only available, if you imported RAW&JPEG pairs. Then you can switch between the RAW file and the JPEG as the original. Is you imported just the RAW files, you do not need this command. Photos will use the RAW file as the original, because the image only has one original file.

Which camera model are you using? Is your Nikon camera on the list of supported cameras for RAW processing? Digital camera RAW formats supported by iOS 12 and macOS Mojave - Apple Support

Photos will never, ever modify the RAW files you import. that is a part of the lossless workflow. When you edit a RAW file in Photos, the RAW file will remain unchanged. Photos is creating a new, edited version from the RAW file. In Photos you will be shown preview of this file. And you can then export this edited version if a new file. But it cannot be exported as a RAW file. RAW files are supposed to be the unmodified data written by the sensor. You can export as a TIFF for the best resolution, or *** JPEG or a PNG.

Photos cannot do that all.

Perhaps the Nikon software that came with your camera can do that. Did you get a copy of Capture NX-D on the CD in the camera box? I have not used a Nikon in a long time. The last Nikon camera I had still used film rolls.