Why I cann't change focus mode to auto?

I have nikon nikkor 50mm AF f1.4d Lens and nikon D3200, focus is always set to MF!

What you don't understand (and it is in the user manual) that you have to use Nikkor AF-S lenses to use the auto-focus. The 50 mm lens you need is the 50 mm AF-S f/1.4G

Any AF lenses you have, you need to manually focus the lens (like we have been doing for over a century). There's an electronic rangefinder you can use to assure that your image is in focus.

In the future, only buy AF-S lenses and you will NOT have this problem again.

Http://www.kenrockwell.com/nikon/compatibility-lens.htm#dslr

I guess you rushed out to buy that camera and lens without checking the memo first.