Miller wants to expand the fields further and asks for a pokémon that knows Rock Smash.
The Request
After you complete the quest Help Wanted: Watering the Fields, talk to Miller at the Farm in Jubilife Village to accept this request.
Get a Pokémon that Can Use Rock Smash
Rock-type and Fighting-type pokémon may be able to learn Rock Smash. Some examples of pokémon that can learn Rock Smash are Machop, Graveler, and Croagunk. You can teach pokémon new moves at the Training Grounds in Jubilife Village.
Reward
After you lend Miller the pokémon, the fields will be expanded. You can switch out the pokémon that you left at the fields by talking to Miller.