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?

I believe diancie respawns directly in the place you catch it.

21 hours ago, Misterdando said:

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?

 

Hello @Misterdando.
Not all Legendaries respawn using the Nikola boss battle. You can find the list of resettable Legendaries by clicking here.
In the case of Diancie, which is not in said list, you should be able to find it in its regular spawn location, which is the Diamond Domain on Love island.

Don't hesitate to contact me if you have further questions or requests.
Have a nice day,
Caelestya