Martia in the Diamond Settlement asks you to get some medicine for Psyduck.

The Request

During the mission Arezu's Predicament, you will reach the Diamond Settlement. Talk to Martia there to accept this request.

Go to Galaxy Hall

Return to Jubilife Village and go into Galaxy Hall. Find Tomma and talk to her to get a dose of Secret Medicine.

Deliver the Medicine

Go back to Crimson Mirelands, then use the map to travel instantly to the Diamond Settlement. Talk to Martia and there will be a cutscene.

Reward

After you deliver the medicine, you get a Max Revive.