Or even:Ok, so the problem is:
Player wants to play a race that doesn't fit in your game world.
There are three easy solutions.
A) Tell the player to play a different race. Sucks for the player, but if you're a bit diplomatic, it should work.
B) Create a niche / unchartered territory / far away continent and place the race there. PC comes from there. Details will be added in when necessary. If nothing else works, use dimensional travel. Problem solved.
C) Make the PC a one-of-a-kind exception. Say, the Dragonborn is the result of an ascension ritual of Bahamut, the Tiefling the result of some demonic curse, the Eladrin a child stolen by fey etc.
D) Use the mechanics and reskin the race. I've seen lots of people reskin dwarves as stout folk/goliaths are mountain folk, etc.