Night Shift & Flux not working on MacBook Air 6,2 (early 2014) since macOS 10.13.2
Go to the 'Display' tab and check the 'Scaled' option instead of 'Default for display' (or vice-versa). Then, simply select whichever text option fits you better.

Once you select one, the display will reset automatically and you'll be having the correct display profile along with Night Shift working again.

Added the solution to Apple forums as well.