Why won’t my Nikon D3400 control the aperture on a 50mm f/1.8D lens?

In order to control the aperture from the D3400 with an older 'D' type lens that has an aperture ring, you need to set the lens' aperture ring to the narrowest (highest f-number) and lock it there. If the aperture ring is in any other position, the camera will not operate the lens' aperture properly.

Keep in mind that there are a few other considerations with using an AF Nikkor 50mm f/1.8D on a D3400 body:

The big one is autofocus. Your D3400 does not have an autofocus motor in the body to drive Nikon 'AF' lenses, which have no focus motor in the lens, but only a mechanical screw drive connection that can be driven by a Nikon body with an in-body AF motor. To use autofocus with a D3400, you need lenses with AF motors in the lens, such as AF-S, AF-I, and AF-P lenses. Some D series lenses are "AF-S", but many more D series lenses are "AF" lenses with no AF motor in the lens. Even if you had a body with a built-in focus motor that can focus the D series AF lenses, the in-lens AF motors of AF-S lenses are usually faster and almost always quieter than a lens that has to be driven by the body's AF motor.

Comparable AF-S lenses are newer designs than their AF series counterparts when looking at the same focal lengths and maximum apertures. They usually have improved optical designs with more low dispersion and aspherical elements that help fight issues such as chromatic aberration and distortion and almost always have better lens coatings that reduce things such as flare and more rounded apertures that give better bokeh when used at wide apertures.

The AF-S Nikkor 50mm f/1.8G is a lens with an AF motor in the lens and a newer optical design when compared to the AF Nikkor 50mm f/1.8D.