i typed in my code and have tried a number of times and it just says "your game cannot receive this gift provided by this code" so I went to the shop and explained and they just gave me another code which still doesn't work.
It is a shiny charizard UK code
The code only works for PAL region Copies.
If you're not in the PAL region, eg if you're in the US or Japan the code won't work.
Or if you have a 3DS and a copy of XY/ORAS that is not from the PAL Region.
E.g A 3DS and a Copy of the Game from the American Region.