Im assuming youre using either B2 or W2, but you cannot get sucker punch on either of those games. You need to get one from either Platinum or HG/SS. So youll have to use platinum, HG, or SS, teach it sucker punch there and then use the poketransfer lab and transfer it to your B2/W2 game.
-For Platinum, you have to go to route 212 and trade it for 6 blue shards and 2 yellow shards.
-For HG/SS, you need to go to the battle frontier and trade it for 40 BP.