Misterdando

A long time ago i had finished the diancie legendary quest. when i got the chance at capturing diancie at the end i made the mistake of not having a masterball and failed in the end to capture diancie. when i tried to battle nikola today (which is my first time trying) in order to get a 2nd chance of capturing diancie, he didnt allow me to fight me saying "it is of no use for you to fight me now". can anyone help?