Check whether Face focusing is on in Live View. I was testing the D500 for use as webcam out of curiosity, checking out the new(ish) capability, and freaked out a bit
when the camera would not focus at all without a face in the scene. Not helped by using a 'focus by wire' lens too, so trying the manual focus overide had no effect.
Also keep in mind, the D500 is not a mirrorless camera, and so does not have the quick focusing in live view mode, compared to mirrorless focusing. In a fast moving scene Live View is not the option to go to.