Human NPC?
 

It seems you cannot choose human, dwarf or elf as an ancestry when building a NPC? is there a reason I cannot use the quick rules the custom ancestry to build a NPC with those ancestries as opposed to build a whole character?

I'm assuming you're not attempting to build an NPC using PC rules, but rather a normal NPC. NPC ancestries aren't the same as PC ancestries, there's no rules for "human" or "dwarf" or "goblin", and the PC versions aren't applicable to NPCs (they're completely different types of things internally because of this, just like how an animal companion ancestry is different than a PC ancestry).

We provide the various premade "ancestries" Paizo provides in Bestiaries and whatnot, but when it comes to NPCs there isn't any baseline statblock for any ancestry. As in there is no "goblin" base that gets worked from, they're all built from the ground up, which is the same as their "add the trait you want and abilities that fit the theme" guidelines for creating new NPCs. "Goblin" is just a trait that makes a thing a goblin, and they might share an ability or two (but not always consistently), but there's no base stats for a goblin NPC. The same is true humans/dwarves/elves that have statblocks in APs. They're *like* PCs, but really, they're all custom built NPCs with a trait label, so unfortunately yeah, the easiest solution is add a custom ancestry, name it what you want, and add the appropriate trait.