On this page of the Assassin's Creed Valhalla Wrath of the Druids game guide you will find a detailed walkthrough of the Sweetening the Pot quest.
By rebuilding the trade post in Ardmel, you will gain access to Honey - one of the main luxury goods in the Wrath of the Druids expansion pack.
Reward for completing the mission: 1,700 XP.
Investigate the Trade Post
Once you have cleared the trade post, read the note lying next to the red tent in the location marked in the image above. You will learn where to look for the document you'll need to rebuild this trade post.
Obtain the deed
Head to the location indicated on the world map and use Odin's Sight to locate the key to the cage. Swim up to the marked spot and dive underwater to get the key.
Return to the shore and open the iron cage, then grab the document and return to the trading post in Ardmel.
Restore the Trade Post
Return to the trade post in Ardmel, interact with the wooden plaque and confirm the construction of the new building. This will complete the Sweetening the Pot quest and you will receive 1,700 XP as a reward.