You must have completed the Teal Mask and the Indigo Disk storyline, claim the mythical pecha berry from the mystery gift, and completed the academy ace tournament at least once, then go to the shop near the Poke Center in Mossui Town in Kitakami. There should be a purple doll in the shop. Interact with the shop and click through the text.
Sources https://www.polygon.com/pokemon-scarlet-violet-guide/24034463/start-dlc-epilogue-catch-pecharunt-mochi-mayhem Experience
