Firmware update failure for AC 1750

> I have been trying to update the firmware on my AC 1750 R6400 [...]

   "R6400" or "R6400v2"?  Look at the product label.

> [...] using Nighthawk. [...]

   "Nighthawk" the app?  I'd try a web browser.

   Visit http://netgear.com/support , put in your (actual) model number,
and look for Downloads.  (For older versions, under Firmware and
Software Downloads, look for "View Previous Versions".)  Find the
kit(s).  Download the kit(s) you want.  Read the "Release Notes" file
for instructions.