Farming requires the right outfit. The DLC introduces the "Ryuusei" outfit series (the swimsuit/summer wear sets) and various costume packs that allow for aesthetic customization. While cosmetic, these packs allow players to further define their protagonist's personality.
If you are playing on PC via emulation, the process is simpler: Rune Factory 5 Switch NSP UPDATE DLC
Compatibility and save-file considerations Farming requires the right outfit