The image grader has nothing to do with Nikon. A few notes that may lend
insight to the issue you’re seeing:
The image grader only works on unsigned 16-bit, single plane FITS
images (the kind that SGPro produces). You can have SGPro produce these
type of FITS images with DSLRs
The image grader does not work with any type of RAW format (either
Canon or Nikon)
The image grader will not work on debayered FITS images if they use
the 3 plane storage method, if your debayered FITS files still use the
single plane method, the image grader will just grade the blue channel.
The image grader will not work if you have opened the image and
resaved it with a different bit depth (i.e. 32 bit).