You need to finish Ophilia’s Fourth Chapter for this Side Story to be unlocked. The Side Story can be found in Wispermill, from the Slender Farmer on the eastern half of the town. It seems like someone is raising the price to use the windmills again, so something needs to be done. There are two solutions for this sidequest, with the first one having you travel to Atlasdam. You’re seeking out Dan, a NPC in front of the weapon shop. Using Primrose or Ophilia, Guide/Allure Dan back to the questgiver to finish.

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Knock some sense into Bryan at Sunshade to finish this Side Story in one way

For the other solution, head to Sunshade and enter the tavern there. There is a man named Bryan on the left side; he’s the one dressed all fancy in front. You will need to Challenge/Provoke Bryan and win the duel, which should be extremely easy, as he only has a strength of four. Once you defeat him, the quest will end.