Hey there,
That NPC is a Move Tutor for Swords Dance; some pokemons can only learn it by TM, which is available at Trainers Valley, as listed in the TM&Move Tutors guide.
Which pokemons did you try with? Make sure that they can actually learn it by Move Tutor, and not only by TM: you can use Bulbapedia's pokemons' movelists to check!
EDIT: Here's the list of all pokemons that can learn Swords Dance by Move Tutor :Shy:
Have a good day
I tried with Garchomp and Haxorus to see if it was a bug, none of them could learn it
