Ron

Lupe, if the OP has an M1 (or better) Macintosh running Mac OS 14 then his hardware and operating system should be more than adequate to run NX Studio. And, version 1.6.1 is the latest version for Mac.

I honestly don't know what the problem could be since I rarely use NX Studio. We do, however, have some Mac users who do use NX Studio extensively. Hopefully, one of them will chime in with a solution.

--Ron

You downloaded the firmware update for the Z7 camera, not the Z7II.

--Ron

You say you want a fixed/prime lens. So, you don't want a zoom lens or variable focus lens? That right?

If that's the case, I would suggest the current Nikkor 50mm AF-S ƒ/1.8G. You can usually find this lens for around $200US new, and often for less than that. It's a perfect fit for your D750. With this lens you'll be able to use all of your camera's features including full or program Auto exposure.

--Ron

The D600 had a problem with spots on images taken with them due to oil or dust in the mirror box. Nikon had a service advisory issued for this camera that supposedly fixed the problem. Nikon would check these cameras and replace the shutter mechanism free of charge if a problem was found. Sadly, the service advisory expired in January of 2020. 

It would probably be a good idea for you to check your camera carefully (while you can still return it) to make sure that it doesn't exhibit this problem. Not all D600's were affected and many of the ones that did were either repaired or replaced by Nikon.

The D610 camera was a direct replacement for the D600 that incorporated the updated shutter mechanism.  

--Ron

Welcome to the forums, Kenneth.

--Ron