It is because you haven't updated your game to version 1.1.read the following steps how to update the game to version 1.1.
1)Go to nintendo e shop.
2)after nintendo e shop is fully loaded,tap the search bar and type the game what what game you have.
4)wait till the update get over.
You might ask what is the benefit of updating,so here it is.
The update is very vague, only mentioning fixing a few bugs and making
for a smoother gameplay experience. You'll need to apply the update in
order to use any of the wireless connectivity features like the PSS,
online trading, battling, or Wonder Trades.
I might not explain properly but you look in the down bellow for proper explanation.