Why is BKT Set settings grayed out on my D50 DSLR camera?

I have a D50 DSLR camera with a Nikkor 18-135mm Lens. I want to turn on the "BKT set" function but the option is grayed out and won't let me access it to turn it on. I went to CSM/Setup Menu and changed it to "Detailed". Any ideas why the BKT function is not accessible?

Do some homework sport - read the book.

It should work if you are in P, A, S, or M. If you are in Auto or a scene mode, then it would be expected to be grayed out.